#include<iostream>
#include<string.h>
using namespace std;
const int N=1e5+10;
const int INF=0x3f3f3f3f;
int n,d,a[N],v[N],ans,mn=INF;
int main(){
	scanf("%d%d",&n,&d);
	memset(a,0x3f,sizeof(a));
	for(int i=1;i<n;i++) scanf("%d",&v[i]);
	for(int i=1;i<=n;i++) scanf("%d",&a[i]);
	for(int i=1;i<=n;i++){
		if(n*d>=v[i]) n-=(n*d-v[i])/d;
		if(a[i]<mn) mn=a[i];
		if(n*d<v[i]) ans+=(v[i]/d-n)*mn,n=0;
	}printf("%d",ans);
	return 0;
}