#include<bits/stdc++.h>
using namespace std;
long long n,d,v[100005],c[100005],minn[100005],ans;
int main(){
	//freopen("road.in","r",stdin);
	//freopen("road.out","w",stdout);
	cin>>n>>d;
	for(int i=2;i<=n;i++){
		cin>>v[i];
		v[i]+=v[i-1];
	}
	minn[0]=INT_MAX;
	int pre=0;
	for(int i=1;i<=n;i++){
		cin>>c[i];
		minn[i]=min(minn[i-1],c[i]);
		int sum=0;
		if((v[i+1]-pre)%d==0){
			sum=(v[i+1]-pre)/d;
		}else{
			sum=(v[i+1]-pre)/d+1;
		}
		pre+=sum*d;
		if(i!=n){
			ans+=minn[i]*sum;
		}
	}
	cout<<ans;
	return 0;
}