#include <bits/stdc++.h>

using namespace std;
using ll = long long;

#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
#define pb push_back
#define ff first
#define sc second
#define int ll

signed main() {
    ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);

    int n, m; cin >> n >> m;
    vector<int> a(n), b(n);
    for (int i = 0; i < n; i++) cin >> a[i];
    for (int i = 0; i < n; i++) cin >> b[i];

    int L = -1, R = 1e18;
    while (R - L > 1) {
        int M = (L + R) / 2;
        int lol = 0;
        for (int i = 0; i < n; i++) {
            if (b[i] >= a[i]) {
                int need = (M + b[i] - 1) / b[i];
                lol += m - need;
            } else {
                if (a[i] * m >= M) {
                    lol += m - (M + a[i] - 1) / a[i];
                } else {
                    int need = (M - a[i] * m + b[i] - 1) / b[i];
                    lol -= need;
                }
            }
            if (lol <= -m * n) {
                break;
            }
        }
        if (lol >= 0) L = M;
        else R = M;
    }
    cout << L << '\n';

    return 0;
}