// 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);
std::ifstream terminal("/dev/tty");
#define PP cerr<<"\033[1;30mpause...\e[0m",terminal.ignore();
#else
#define debug(x...)
#define Debug(x...)
#define PP
#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 NaN (int)(0x8b88e1d0595d51d1)
#define double long double
#define int long long
#define uint unsigned long long
#define MAXN 200010

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

    int n, m; cin >> n >> m;
    vector<vector<int>> a(n, vector<int>(n, 0));
    vector<vector<int>> e(n);
    rep(i, 0, m) {
        int u, v; cin >> u >> v; --u, --v;
        e[u].push_back(v), e[v].push_back(u);
        a[u][v] = a[v][u] = 1;
    }
    auto bfs = [&](int s, int t) -> int {
        vector<int> dis(n, INF); dis[s] = 0;
        queue<int> q; q.push(s);
        while (!q.empty()) {
            int u = q.front(); q.pop();
            for (auto v: e[u]) {
                if (!a[u][v] || dis[v] != INF) continue;
                dis[v] = dis[u] + 1;
                q.push(v);
                if (v == t) return dis[v];
            }
        }
        return INF;
    };
    int res = INF;
    rep(i, 0, n) rep(j, i+1, n) {
        if (!a[i][j]) continue;
        a[i][j] = a[j][i] = 0;
        cmin(res, bfs(i, j) + 1);
        a[i][j] = a[j][i] = 1;
    }
    cout << (res!=INF ? res : -1) << endl;

    return 0;
}