#include <iostream>
#include <vector>
using namespace std;
bool check(long long min_lvl, int m, vector<int> &A, vector<int> &B) {
long long cnt = 0;
int n = A.size();
for (int i = 0; i < n; ++i) {
int a = max(A[i], B[i]);
int b = B[i];
long long curr_cnt = min((long long)m, (min_lvl + a - 1) / a);
cnt += curr_cnt + max((long long)0, (min_lvl - curr_cnt * a + b - 1) / b);
if (cnt > (long long)n * m)
return false;
}
return true;
}
long long BS(int m, vector<int> &A, vector<int> &B) {
long long lo = 0, hi = 1e18;
while (lo < hi) {
long long mid = (lo + hi + 1) / 2;
if (check(mid, m, A, B))
lo = mid;
else
hi = mid - 1;
}
return lo;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
vector<int> A(n), B(n);
for (auto &a : A)
cin >> a;
for (auto &b : B)
cin >> b;
cout << BS(m, A, B) << '\n';
return 0;
}
