#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int ll
#define pb push_back
const int MAX_N = 3e5;
int a[MAX_N + 5], b[MAX_N + 5];
bool check (int mij, int n, int m) {
int i, sum, val, num_curs;
sum = i = 0;
num_curs = m * n;
while (i < n && sum <= num_curs) {
val = max(a[i], b[i]);
sum++;
if (val * m >= mij) {
sum += ((mij - 1) / val);
} else {
if (val == b[i]) {
sum += ((mij - 1) / b[i]);
} else {
sum += (m + (mij - val * m - 1) / b[i]);
}
}
i++;
}
return (sum <= num_curs);
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, m, i, st, dr, mij;
cin >> n >> m;
for (i = 0; i < n; i++) {
cin >> a[i];
}
for (i = 0; i < n; i++) {
cin >> b[i];
}
st = 0, dr = LLONG_MAX;
while (st <= dr) {
mij = (st + dr) / 2;
if (check(mij, n, m)) {
st = mij + 1;
} else {
dr = mij - 1;
}
}
cout << (st == 0 ? 0 : st - 1);
return 0;
}
