#include<bits/stdc++.h>
using namespace std;
int main() {
//	freopen("road.in","r",stdin);
//	freopen("road.out","w",stdout);
	long long n,d,a[10010],s[10010],m=0,x=INT_MAX,y,j,p=0;
	scanf("%lld %lld",&n,&d);
	for(int i=1;i<n;i++) scanf("%lld",&a[i]);
	for(int i=1;i<=n;i++) scanf("%lld",&s[i]);
	for(int i=1;i<n;i++){
		x=min(x,s[i]);
		p+=a[i];
		if(p>0)
		{
            m+=(p+d-1)/d*x;
            p-=(p+d-1)/d*d;
        }
	}
	printf("%lld",m);
	return 0;
}