#include <algorithm>
#include <iostream>
#define int long long
using namespace std;
const int MAXN = 300005;
int n, m;
int a[MAXN];
int b[MAXN];
void init() {
cin >> n >> m;
for (int i = 0; i < n; i++) cin >> a[i];
for (int i = 0; i < n; i++) cin >> b[i];
for (int i = 0; i < n; i++) a[i] = max(a[i], b[i]);
}
int div_ceil(int x, int y) {
int ans = x / y;
if (x % y > 0) ans++;
return ans;
}
bool check(int D) {
__int128 extra = 0;
// cout << "D = " << D << '\n';
for (int i = 0; i < n; i++) {
if (a[i] * m >= D) {
extra += m - div_ceil(D, a[i]);
} else { // a[i] * m < D
extra -= div_ceil(D - a[i] * m, b[i]);
}
}
return extra >= 0;
}
void solve() {
int l = 1;
int r = 1 + (int)1e18;
// check(D) : D 愈小愈容易
while (l != r) {
int mid = (l + r) / 2;
if (check(mid) == false) {
r = mid;
} else {
l = mid + 1;
}
}
int ans = r - 1;
cout << ans << '\n';
}
signed main() {
cin.tie(0);
cin.sync_with_stdio(0);
init();
solve();
return 0;
}
