#include <bits/stdc++.h>
using namespace std;
const long long MAX_ANSWER = 2e18 + 5;
void solve() {
int n, m;
cin >> n >> m;
vector<int> a(n), b(n);
for (int &v : a) {
cin >> v;
}
for (int &v : b) {
cin >> v;
}
if (m == 1) {
int ans = INT_MAX;
for (int i = 0; i < n; i++) {
ans = min(ans, max(a[i], b[i]));
}
cout << ans << "\n";
} else {
long long l = -1, r = MAX_ANSWER;
while (r > l + 1) {
long long mid = (l + r) / 2;
long long req = 0;
for (int i = 0; i < n; i++) {
req += (mid + b[i] - 1) / b[i];
req = min(req, MAX_ANSWER);
}
if (req > 1ll * n * m) {
r = mid;
} else {
l = mid;
}
}
cout << l << "\n";
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
solve();
}
