#include <bits/stdc++.h>
using namespace std;
#ifdef Ramzi
#include "debug.h"
#else
#define debug(...)
#endif
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, m;
cin >> n >> m;
vector<int64_t> a(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
}
vector<int64_t> b(n);
for (int i = 0; i < n; i++) {
cin >> b[i];
}
auto check = [&](int64_t mid) -> bool {
int64_t tot = n * m;
for (int i = 0; i < n; i++) {
int64_t s = 0;
if (a[i] > b[i]) {
int64_t req = (mid + a[i] - 1) / a[i];
if (req <= m) {
tot -= req;
} else {
int64_t rem = mid - m * a[i];
tot -= m;
tot -= (rem + b[i] - 1) / b[i];
}
} else {
int64_t req = (mid + b[i] - 1) / b[i];
tot -= req;
}
if (tot < 0) {
return false;
}
}
return true;
};
int64_t lo = 0, hi = int64_t(1e18), ans = 0;
while (lo <= hi) {
int64_t mid = (lo + hi) >> 1;
if (check(mid)) {
ans = mid;
lo = mid + 1;
} else {
hi = mid - 1;
}
}
cout << ans << "\n";
return 0;
}
