code

#include<bits/stdc++.h>
using namespace std;
int d[100010],a[100010];
int main(){
	int n,m;
	scanf("%d%d",&n,&m);
	for(int i=2;i<=n;++i){
		scanf("%d",&d[i]);
	} 
	for(int i=1;i<=n;++i){
		scanf("%d",&a[i]);
	}
	int ans=0;
	int cnt=0;
	int mn=a[1];
	int pos=1;
	for(int i=2;i<=n;++i){
		ans+=d[i];
		if(a[i]<mn){
			mn=a[i];
			if(ans%m!=0)cnt+=((ans/m)+1)*a[pos];
			else cnt+=(ans/m)*a[pos];
			printf("cnt=%d\n",cnt);
			ans=0;
		}
	}
	if(ans%m!=0)cnt+=((ans/m)+1)*a[pos];
	else cnt+=(ans/m)*a[pos];	
	printf("%d",cnt);
	return 0;
}