莫名其妙的WA，大样例能过。

#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
long long n,d,a[N],b[N],now,last,ans;
int main(){
	//freopen("road.in","r",stdin);
	//freopen("road.out","w",stdout);
	cin>>n>>d;
	for(int i=1;i<n;i++) cin>>a[i];
	for(int i=1;i<=n;i++) cin>>b[i];
	b[0]=1e9;
	for(int i=1;i<n;i++){
		int nowChoose;
		if(b[i]>b[last]) nowChoose=last;
		else{
			last=i;
			nowChoose=i;
		}
		long long Ned=a[i]-now;
		now=0;
		ans+=(Ned/d)*b[nowChoose];
		if(Ned%d>0){
			now+=d-(Ned-Ned/d*d);
			ans+=b[nowChoose];
		}
	}
	cout<<ans<<endl;
	return 0;
}