// 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
 
struct lichao {
    struct line {
        int k, b;
        line(): k(NaN), b(NaN) {} 
        line(int k_, int b_): k(k_), b(b_) {}
        int operator()(int x) {return k*x + b; }
        int exist() {return k != NaN; }
        void clear() {k = NaN, b = NaN; }
    };
    struct node {
        int l, r;
        int val;
        node *ch[2];
        line tag;
    };
    vector<node> pool;
    int total = 0;
    node *root;
    node* newnode(int l, int r) {
        node *x = &pool[total++];
        x->l = l, x->r = r, x->val = INF;
        return x;
    }
    lichao(int L, int R) {
        pool.resize((R-L+1) << 1);
        function<void(node*)> build = [&](node *x) -> void {
            if (x->l == x->r) return;
            int mid = (x->l+x->r) >> 1;
            build(x->ch[0] = newnode(x->l, mid));
            build(x->ch[1] = newnode(mid+1, x->r));
        };
        build(root = newnode(L, R));
    }
    void update(node *x, line l) {
        if (!x) return;
        if (!x->tag.exist()) {
            x->tag = l;
            return;
        }
        int mid = (x->l+x->r) >> 1;
        cmin(x->val, x->tag(mid));
        if (l(x->l) <= x->tag(x->l) && l(x->r) <= x->tag(x->r)) x->tag = l;
        else if (l(x->l) >= x->tag(x->l) && l(x->r) >= x->tag(x->r)) return;
        else update(x->ch[0], l), update(x->ch[1], l);
    }
    void insert(int l, int r, int k, int b) {
        function<void(node*)> dfs = [&](node *x) -> void {
            if (x->l > r || x->r < l) return;
            if (x->l >= l && x->r <= r) {
                update(x, line(k, b));
                return;
            }
        };
        dfs(root);
    }
    int query(int p) {
        int res = INF;
        for (node *x = root; x; x = x->ch[p > ((x->l+x->r) >> 1)]) {
            if (x->tag.exist()) cmin(res, x->tag(p));
        }
        return res;
    }
    void graphviz_dump(string filename = "graph.dot") {
        FILE *f = fopen(filename.c_str(), "w");
        fprintf(f, "digraph {\n");
        rep(i, 0, total) {
            node *x = &pool[i];
            fprintf(f, "    %lld;\n");
        }
        rep(i, 0, total) {
            node *x = &pool[i];
            if (x->ch[0]) fprintf(f, "    %lld -> %ld[arrowhead=\"none\"];\n", i, x->ch[0] - &pool[0]);
            if (x->ch[1]) fprintf(f, "    %lld -> %ld[arrowhead=\"none\"];\n", i, x->ch[1] - &pool[0]);
        }
        fprintf(f, "}\n");
    }
};
 
int32_t main() {
    ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
 
    int n, F; cin >> n >> F;
    vector<int> s(n), f(n);
    rep(i, 0, n) cin >> s[i];
    rep(i, 0, n) cin >> f[i];
    int N = s.back();
    lichao t(1, N);
    t.insert(1, N, F, 0);
    rep(i, 0, n-1) {
        int best = t.query(s[i]);
        t.insert(1, N, f[i], best);
    }
    cout << t.query(N) << endl;
 
    return 0;
}