#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
const  int N=1e6+5;
int n,sum[N],youjia,cnt,ans,x,youliang,that,d;
int main(){
//	freopen("road.in","r",stdin);
//	freopen("road.out","w",stdout);
	scanf("%d %d",&n,&d);
	for(int i=1;i<n;i++)
		scanf("%d",&x),sum[i+1]=sum[i]+x;
	//scanf("%d",&youjia);
	x=0x3f3f3f3f;
	cnt=0;
	for(int i=1;i<=n;i++){
		scanf("%d",&youjia);
		if(youjia<x){
			int awaa=sum[i]-sum[cnt],c=0;
			awaa-=youliang;
			youliang=awaa%d;
			c=awaa/d*x;
			if(awaa%d)
				c+=x;
			ans+=c;
			cnt=i;
			x=youjia;
		}
	}
	int awaa=sum[n]-sum[cnt],c=0;
	awaa-=youliang;
	youliang=awaa%d;
	c=awaa/d*x;
	if(awaa%d)
		c+=x;
	ans+=c;
	x=youjia;
	printf("%d",ans);
	return 0;
}/*
5 4
10 10 10 10
9 8 9 6 5
answer:
79
*/