#include<iostream>
#include<cmath>
using namespace std;
int v[100010],vis[100010],a[100010],n,d;
int main(){
	cin>>n>>d;
	for(int i=1;i<=n-1;i++) cin>>v[i];
	for(int i=1;i<=n;i++) cin>>a[i];
	int now=2147483647,ans=0;
	for(int i=1;i<n;i++){
		now=min(a[i],now);
		ans+=(ceil((v[i])*1.0*now/d));
	}
	cout<<ans;
	return 0;
}