#include <bits/stdc++.h>
using namespace std;
#define int long long
#define vi vector<int>
#define show3(x,y,z) cout<<#x<<": "<<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl;
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n,m;
cin>>n>>m;
vector<int> a(n),b(n);
for(auto &x:a)cin>>x;
for(auto &x:b)cin>>x;
int lo=1,hi=1e17;
while(lo<hi){
int mid=(lo+hi+1)/2; // min level is mid
int over=0;
for(int i=0;i<n;++i){
// try to get a level of >= mid
int take=0;
if(a[i]>b[i]){ // if its better to go for the course
take+=min(m,(mid-1)/a[i] + 1);
}
// cout<<take<<endl;
int left=mid-take*a[i];
if(left>0){
// use as many b[i] to reach mid
take+=((left-1)/b[i] + 1);
}
// cout<<take<<endl;
over+=m-take;
}
// show3(mid,over,lo);
if(over<0)hi=mid-1;
else lo=mid;
}
cout<<lo;
}
// 27077044488345 (wrong)
// 41397427274960
