/*
author : PakinDioxide
created : 28/03/2025 20:10
task : JOI22_ho_t2
*/
#include <bits/stdc++.h>
#define ll unsigned long long
using namespace std;
int main() {
ios::sync_with_stdio(0), cin.tie(0);
ll n, m;
cin >> n >> m;
ll a[n], b[n];
for (auto &e : a) cin >> e;
for (auto &e : b) cin >> e;
ll l = 0, r = LLONG_MAX, ans = 0;
while (l <= r) {
ll x = l + (r-l)/2, cnt = 0;
for (int i = 0; i < n; i++) {
if (a[i] >= b[i]) {
if (a[i] * m >= x) cnt += ceil((double) x / a[i]);
else cnt += m + ceil((double) (x - a[i] * m) / b[i]);
} else cnt += ceil((double) x / b[i]);
}
if (cnt <= n*m) ans = x, l = x+1;
else r = x-1;
}
cout << ans << '\n';
}
