#include<bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
ll a[n], b[n];
for(int i = 0; i < n; i++) {
cin >> a[i];
}
for(int i = 0; i < n; i++) {
cin >> b[i];
a[i] = max(b[i], a[i]);
}
ll l = 0, r = 1e18 + 1;
while(l < r) {
ll mid = (l + r + 1) / 2;
ll cnt = 0;
for(int i = 0; i < n; i++) {
if(mid <= a[i] * m) cnt += mid / a[i] + min(1LL, mid % a[i]);
else cnt += (mid - a[i] * m) / b[i] + min(1LL, (mid - a[i] * m) % b[i]) + m;
}
if(cnt <= 1LL * n * m) l = mid;
else r = mid - 1;
}
cout << l << '\n';
return 0;
}
