#include<bits/stdc++.h>
#define int long long
using namespace std;
int n,m;
vector<int> a,b;
int dv(int x,int y){
if(x<=0) return 0;
if(y*(x/y)==x) return x/y;
else return x/y+1;
}
bool check(int x){
int emp=0,ext=0;
for(int i=0;i<n;i++){
if(x<=m*a[i]) emp+=dv(x,a[i]);
else{
ext+=dv(x-m*a[i],b[i]);
emp+=m;
}
}//cout<<x<<' '<<ext<<' '<<emp<<' '<<n*m<<endl;
return n*m>=ext+emp;
}
signed main(){
cin>>n>>m;
a.resize(n);
b.resize(n);
for(int &i:a) cin>>i;
for(int i=0;i<n;i++){
cin>>b[i];
a[i]=max(a[i],b[i]);
}
int l=0,r=1e18+100,mid;
while(r-l>1){
mid=(l+r)/2;
if(check(mid)) l=mid;
else r=mid;
}
cout<<l<<endl;
}
