#include <bits/stdc++.h>
#define int long long
#define sz(v) (int)v.size()
using namespace std;
const int maxn = (int)3e5 + 5;
const int INF = (int)1e18 + 5;
int n, m;
int a[maxn], b[maxn];
bool check(int x) {
int rem = 0;
for (int i = 1; i <= n; i++) {
if (a[i] * m >= x) {
rem += m - ((x + a[i] - 1) / a[i]);
} else {
rem -= (x - a[i] * m + b[i] - 1) / b[i];
}
}
return rem >= 0;
}
void solve() {
cin >> n >> m;
for (int i = 1; i <= n; i++) cin >> a[i];
for (int i = 1; i <= n; i++) cin >> b[i];
for (int i = 1; i <= n; i++) a[i] = max(a[i], b[i]);
int l = 1, r = (int)2e18, mid;
while (l < r) {
mid = (l + r + 1) / 2;
if (check(mid)) l = mid;
else r = mid - 1;
}
cout << l << '\n';
}
signed main() {
ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
solve();
return 0;
}
