#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0);cin.tie(0);
#define s second
#define f first
typedef long long ll;
const ll MOD = 1e9 + 7;
const ll LOGN = 20;
const ll MAXN = 3e5 + 100;
vector<ll> A, B;
ll N, M;
bool check(ll x) {
ll need = 0;
for (int i = 1; i <= N; i++) {
if (M * A[i] >= x)
need += (x + A[i] - 1) / A[i];
else
need += M + (x - A[i] * M + B[i] - 1) / B[i];
if (need > N * M)
return false;
}
return (need <= N * M);
}
int main() {
fast
cin >> N >> M;
A = vector<ll>(N+1);
B = vector<ll>(N+1);
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]);
ll L = 0;
ll R = 2 * 1e18;
ll ans = -1;
while (L <= R) {
ll mid = L + (R-L)/2;
if (check(mid)) {
L = mid + 1;
ans = mid;
} else
R = mid - 1;
}
cout << ans << "\n";
}
