#include "bits/stdc++.h"
#define int long long
using namespace std;
template <typename _T>
bool chmin(_T &a, const _T &b) {
if (a > b) {
a = b;
return true;
}
return false;
}
template <typename _T>
bool chmax(_T &a, const _T &b) {
if (a < b) {
a = b;
return true;
}
return false;
}
signed main() {
cin.tie(nullptr)->sync_with_stdio(false);
int n, m;
cin >> n >> m;
vector<int> a(n);
for(auto &x : a){
cin >> x;
}
vector<int> b(n);
for(auto &x : b){
cin >> x;
}
for(int i = 0; i < n; i++){
a[i] = max(a[i], b[i]);
}
auto check = [&](int val) {
int cnt = 0;
for (int i = 0; i < n; i++) {
if (a[i] * m >= val) {
cnt += ((val - 1) / a[i] + 1);
} else {
if (val > m * a[i]) {
cnt += (m + (val - m * a[i] - 1) / b[i] + 1);
} else {
cnt += m;
}
}
if (cnt > m * n) return false;
}
return cnt <= m * n;
};
int l = 0, r = 2e18;
while (l + 1 < r) {
int mid = l + (r - l) / 2;
if (check(mid)) {
l = mid;
} else {
r = mid;
}
}
cout << l << "\n";
}
