#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll n, m;
bool check(vector<ll> &a, vector<ll> &b, ll K) {
ll sm = 0;
for (ll i = 0; i < n; i++) {
if (a[i] > b[i]) {
if (m*a[i] < K) {
sm += m;
ll tmp = K; tmp -= m*a[i];
sm += ((tmp/b[i]) + ((tmp%b[i]) > 0));
} else {
sm += ((K/a[i]) + ((K%a[i]) > 0));
}
} else {
sm += ((K/b[i]) + ((K%b[i]) > 0));
}
}
return sm <= m*n;
}
int main() {
cin >> n >> m;
vector<ll> a(n);
vector<ll> b(n);
for (auto &it: a) cin >> it;
for (auto &it: b) cin >> it;
ll lo = 0;
ll hi = 2*1e18;
// cout << check(a, b, 18) << "\n";
while (lo < hi) {
ll mid = lo + (hi - lo)/2;
if (check(a, b, mid)) {
lo = mid+1;
} else {
hi = mid;
}
}
if (!check(a, b, lo)) --lo;
cout << lo;
}
