#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int MAXN = 3e5 + 10;
const ll INF = 1e18;
ll a[MAXN], b[MAXN];
ll n, m;
bool check(ll x){
ll cnt = 0;
for(int i=1; i<=n; i++){
if(b[i] >= a[i]){
cnt += (x + b[i] - 1) / b[i];
} else{
ll cur = min(m, (x + a[i] - 1) / a[i]);
ll left = max(0LL, x - cur * a[i]);
cur += (left + b[i] - 1) / b[i];
cnt += cur;
}
if(cnt > n * m) return false;
}
return cnt <= n * m;
}
ll bs(){
ll l = 0, r = INF;
while(l < r){
ll m = l + (r - l + 1) / 2;
if(check(m)) l = m;
else r = m - 1;
}
return l;
}
int main(){
cin.tie(0)->sync_with_stdio(0);
cin >> n >> m;
for(int i=1; i<=n; i++) cin >> a[i];
for(int i=1; i<=n; i++) cin >> b[i];
cout << bs() << "\n";
return 0;
}
