// compile: make data
// run: ./data < data.in
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifdef LOCAL
#include <debug/codeforces.h>
#define debug(x...) _debug_print(#x, x);
#define Debug(x...) _debug_print_format(#x, x);
#else
#define debug(x...)
#define Debug(x...)
#endif
template<typename...Args> void print_(Args...args){((cout<<args<<" "),...)<<endl;}
#define rep(i,a,b) for(int i=(a);i<(int)(b);++i)
#define sz(v) ((int)(v).size())
#define print(...) print_(__VA_ARGS__);
#define FIND(a, x) ((find(a.begin(),a.end(),(x))!=a.end())?1:0)
#define cmin(x,...) x=min({(x), __VA_ARGS__})
#define cmax(x,...) x=max({(x), __VA_ARGS__})
#define INTMAX (int)(9223372036854775807)
#define INF (int)(1152921504606846976)
#define double long double
#define int long long
#define MAXN 200010

struct graph {
    struct node {
        int u, v, w;
        bool operator<(const node &other) const {
            return w < other.w;
        }
        bool operator==(const node &other) const {
            return u == other.u && v == other.v && w == other.w;
        }
    };
    vector<vector<node>> e;
    int n;
    bool directed;
    bool weighted = 0;
    graph(int V, bool D = 0) {
        n = V;
        e.resize(n);
        directed = D;
    }
    void add_edge(int u, int v, int w = 1) {
        e[u].push_back(node{u, v, w});
    }
    int spfa(int s, vector<int> &dis, vector<int> &pre) {
        dis.resize(n, INF);
        pre.resize(n, -1);
        vector<bool> inq(n, 0);
        vector<int> cnt(n, 0);
        queue<int> q;
        dis[s] = 0, q.push(s), inq[s] = 1;
        while (!q.empty()) {
            int u = q.front(); q.pop();
            inq[u] = 0;
            for (auto edge: e[u]) {
                int v = edge.v, w = edge.w;
                if (dis[u] != INF && dis[v] > dis[u] + w) {
                    pre[v] = u;
                    dis[v] = dis[u] + w;
                    if (!inq[v]) {
                        q.push(v), inq[v] = 1, ++cnt[v];
                        if (cnt[v] >= n) return v;
                    }
                }
            }
        }
        return -1;
    }
    void dijkstra(int s, vector<int> &dis) {
        dis.resize(n, INF);
        vector<bool> vis(n, 0);
        priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
        dis[s] = 0, pq.push({0, s});
        while (!pq.empty()) {
            int u = pq.top().second; pq.pop();
            if (vis[u]) continue;
            vis[u] = 1;
            for (auto edge: e[u]) {
                int v = edge.v, w = edge.w;
                if (dis[v] > dis[u] + w) {
                    dis[v] = dis[u] + w;
                    pq.push({dis[v], v});
                }
            }
        }
    }
    void floyd(vector<vector<int>> &dis) {
        dis.resize(n, vector<int>(n, INF));
        rep(i, 0, n) dis[i][i] = 0;
        rep(u, 0, n) for (auto edge: e[u]) {
            int v = edge.v, w = edge.w;
            dis[u][v] = min(dis[u][v], w);
        }
        rep(k, 0, n) rep(i, 0, n) rep(j, 0, n) {
            if (dis[i][k] == INF || dis[k][j] == INF) continue;
            dis[i][j] = min(dis[i][j], dis[i][k] + dis[k][j]);
        }
    }
    void graphviz_dump(string filename = "graph.dot") {
        ofstream gf;
        gf.open(filename);
        gf << (directed ? "digraph" : "graph") << " {\n";
        gf << "    "; rep(i, 0, n) gf << i << " ;"[i==n-1]; gf << endl;
        string notation = directed ? " -> " : " -- ";
        for (auto es: e) for (auto edge: es) if (edge.w != 1) weighted = 1;
        for (auto es: e) {
            for (auto edge: es) {
                if (!directed && edge.u > edge.v) continue;
                gf << "    " << edge.u << notation << edge.v << (weighted ? " ;\n" : ";\n");
            }
        }
        gf << "}\n";
    }
    int dfs(int u, vector<int> &vis, vector<int> &instack, vector<int> &pre) {
        instack[u] = vis[u] = 1;
        for (auto [_, v, __]: e[u]) {
            if (instack[v]) {
                pre[v] = u;
                return v;
            }
            if (vis[v]) continue;
            pre[v] = u;
            int rtn = dfs(v, vis, instack, pre);
            if (~rtn) return rtn;
        }
        instack[u] = 0;
        return -1;
    }
};

int32_t main() {
    ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);

    int n, m; cin >> n >> m;
    graph g(n, 1);
    rep(i, 0, m) {
        int u, v; cin >> u >> v;
        g.add_edge(u-1, v-1);
    }
    vector<int> pre(n, -1), vis(n, 0), instack(n, 0);
    int lst = -1;
    rep(i, 0, n) {
        if (vis[i]) continue;
        lst = g.dfs(i, vis, instack, pre);
        if (~lst) break;
    }
    if (!~lst) {
        cout << "IMPOSSIBLE" << endl;
        return 0;
    }
    vector<int> v, inq(n, 0);
    for (int x = lst; ; x = pre[x]) {
        v.push_back(x);
        if (inq[x]) break;
        inq[x] = 1;
    }
    cout << sz(v) << endl;
    for (int i = sz(v)-1; i >= 0; --i) {
        cout << v[i]+1 << " \n"[i==0];
    }

    return 0;
}