#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 3e5+10;
ll a[N], b[N];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
ll n, m;
cin >> n >> m;
for(int i = 1; i <= n; ++i) cin >> a[i];
for(int i = 1; i <= n; ++i) cin >> b[i];
ll l = 0, r = 3e18, ans;
while(l < r){
ll mid = (l+r)>>1;
ll needed = 0, extra = 0;
for(int i = 1; i <= n; ++i){
if(mid <= max(a[i], b[i])*m){
extra += m-ceil(1.0*mid/max(a[i], b[i]));
}else{
needed += ceil((1.0*mid-m*max(a[i], b[i]))/b[i]);
}
}
if(extra >= needed) l = mid+1, ans = mid;
else r = mid;
}
cout << ans << '\n';
}
