公路

#include <bits/stdc++.h>
using namespace std;
const int N=100010;
long long a[N],b[N];
int main(){
freopen("road.in","r",stdin);
freopen("road.out","w",stdout);
int n;
cin>>n;
long long d;
cin>>d;
for(int i=2;i<=n;i++) cin>>a[i];
for(int i=1;i<=n;i++) cin>>b[i];
long long ans=0,p=0,sum=0,oil=1e9;
for(int i=2;i<=n;i++){
sum+=a[i];
oil=min(oil,b[i-1]);
long long c=(sum-p+d-1)/d;
ans+=coil;
p+=cd;
}
cout<<ans;
fclose(stdin);
fclose(stdout);
return 0;
}