#include<cstdio>
#include<iostream>
using namespace std;
long long n,d,v[114514],a[114514],cheap=11451419,fuel=0,money=0;
int main(){
freopen("road.in","r",stdin);
freopen("road.out","w",stdout);
cin>>n>>d;
for(int q=1;q<n;++q){
cin>>v[q];
}
for(int w=1;w<=n;++w){
cin>>a[w];
}
for(int e=1;e<n;++e){
if(a[e]<cheap) cheap=a[e];
while(fuel<v[e]){
fuel+=d;
money+=cheap;
}
fuel-=v[e];
}
cout<<money;

不要相信int，也不要相信0x3f

三年oi一场空，不开long long 见祖宗