#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pll pair<int, int>
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define endl '\n'
#define ld long double
#define sz(x) static_cast<int>((x).size())
#define i5 tuple<int,int,int,int,int>
int cdiv(int a, int b){
return a/b + !!(a%b);
}
signed main(){
int n,m;cin>>n>>m;
int a[n], b[n]; for(int i=0;i<n;i++)cin>>a[i];
for(int i=0;i<n;i++){
cin>>b[i];
if(a[i] < b[i])a[i]=b[i];
}
const int mx=n*m;
int l=1, r=1e18, mid;
while(r-l>1){
mid=(l+r)/2;
int req=0, add=0;
for(int i=0;i<n;i++){
if(cdiv(mid, a[i]) > m){ // overflow
int need=cdiv(mid-m*a[i], b[i]);
req+=need;
if(req > mx){
req=mx+1;
break;
}
}
else {
add+=m-cdiv(mid,a[i]);
}
}
if(req <= add){
l=mid;
}
else {
r=mid;
}
}
cout<<l;
}
