#include<bits/stdc++.h>
using namespace std;
#define int long long
int f(int a, int b){
return (a + b - 1) / b;
}
void solve(){
int n, m; cin >> n >> m;
vector<int> a(n + 1), b(n + 1);
for(int i = 1; i <= n; i++) cin >> a[i];
for(int i = 1; i <= n; i++) cin >> b[i];
auto ok = [&](int mid){
int cnt = 0;
vector<int> trash;
for(int i = 1; i <= n; i++){
if(m * max(a[i], b[i]) >= mid) cnt += m - f(mid, max(a[i], b[i]));
else trash.push_back(i);
}
for(int i: trash) cnt -= f(mid - m * max(a[i], b[i]), b[i]);
return (cnt >= 0);
};
int l = 1, r = 1e18, res = 1;
while(l <= r){
int mid = (l + r) / 2;
if(ok(mid)) l = mid + 1, res = mid;
else r = mid - 1;
}
cout << r;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
solve();
}
