#include<bits/stdc++.h>
#define int long long
using namespace std;
int a[300005], b[300005];
bool check(int n, int m, int x)
{
int re = 0;
for(int i = 1; i <= n; i++) if(x <= m*a[i]){
re += m - (x+a[i]-1)/a[i];
}
for(int i = 1; i <= n; i++) if(x > m*a[i]){
int num = x; num -= m*a[i];
re -= (num+b[i]-1)/b[i];
}
return re >= 0;
}
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 = 1, r = 1e18, ans = 0;
while(l <= r){
int mid = (l+r)/2;
if(check(n, m, mid) == 1){ans = mid; l = mid+1;}
else r = mid-1;
}
cout<<ans;
}
