#include <iostream>
using namespace std;
#define int long long
int a[3<<17], b[3<<17];
signed main(){
int 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], a[i] = max(a[i], b[i]);
int l = 0, r = 1.1e18;
while (l + 1 < r){
int mid = (l + r) / 2;
int req = 0;
for (int i=1;i<=n;i++){
if (m * a[i] >= mid)
req += (mid + a[i] - 1) / a[i];
else
req += m + (mid - m * a[i] + b[i] - 1) / b[i];
if (req > n * m + 1)
break;
}
if (req <= m * n)
l = mid;
else
r = mid;
}
cout<<l<<'\n';
}
