#include <bits/stdc++.h>
#define int long long
using namespace std;
using idata = vector<int>;
int N, M;
idata As, Bs;
int ceilDiv (int x, int y) {
return (x + y - 1) / y;
}
int solve (int A, int B, int target) {
if (A < B) A = B;
if (A * M >= target)
return M - ceilDiv(target, A);
int local = target - A * M;
return - ceilDiv(local, B);
}
bool check (int target) {
int res = 0;
for (int i = 0; i < N; i ++) {
res += solve(As[i], Bs[i], target);
if (res < - M * N)
return false;
}
return res >= 0;
}
signed main () {
cin >> N >> M;
if (N < 0) {
N *= -1;
int a, b;
cin >> a >> b;
As.resize(N, a);
Bs.resize(N, b);
} else {
As.resize(N); Bs.resize(N);
for (int i = 0; i < N; i ++) cin >> As[i];
for (int i = 0; i < N; i ++) cin >> Bs[i];
}
int a = 0;
int b = 2e18;
while (b - a > 1) {
int c = (a + b) >> 1;
if (check(c)) a = c;
else b = c;
}
cout << a << endl;
}
