#include<bits/stdc++.h>
using namespace std;
int a[100100],b[100010];
int main()
{
/*	freopen("apple.in","r",stdin);
	freopen("apple.out","w",stdout);*/
	int n,m,s=0;
	scanf("%d%d",&n,&m);
	int cnt=0,sum=0,mi=2e9;
	for(int i=1;i<=n-1;i++)
		scanf("%d",&b[i]);
	for(int i=1;i<=n;i++){
		scanf("%d",&a[i]);
		mi=min(mi,a[i]);
		if((b[i]-s)%m==0)
		{
			sum=sum+mi*(b[i]-s)/m;
			s=0;
		}
		else
		{
			sum=sum+((b[i]-s)/m+1)*mi;
			s=((b[i]-s)/m+1)*m-b[i];
		}
	}
	cout<<sum;
	return 0;
}