#include<bits/stdc++.h>
using namespace std;
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
long long n,m;cin>>n>>m;
vector<long long>arr(n);
for (int i = 0;i<n;++i)cin>>arr[i];
vector<long long>brr(n);
for (int i = 0;i<n;++i)cin>>brr[i];
long long left = 0,right = 1e17;
long long ans = 0;
while(left<=right){
long long mid = (left + right)>>1;
bool okay = true;
long long chance = 0;
for (int i = 0;i<n;++i){
long long temp = (mid + max(arr[i],brr[i]) - 1)/max(arr[i],brr[i]);
chance+=max(0LL,m - temp);
}
for (int i = 0;i<n;++i){
if ((mid + m - 1) / m > max(arr[i],brr[i])){
long long cur = m * max(arr[i],brr[i]);
long long temp = min(chance,(mid - cur + brr[i] - 1) / brr[i]);
cur+=temp * brr[i];
if (cur < mid)okay = false;
chance-=temp;
}
}
if (okay){
left = mid + 1;
ans = mid;
}
else{
right = mid - 1;
}
}
cout<<ans<<'\n';
return 0;
}
