#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll n,d,v[101010];
ll o,ans,minn=1e18,a;
int main(){
	freopen("road.in","r",stdin);
	freopen("road.out","w",stdout);
	cin>>n>>d;
	for(int i=2;i<=n;i++) cin>>v[i];
	for(int i=1;i<=n;i++){
		cin>>a;
		o-=v[i];
		if(o<0){
			ll lo=(d-o-1)/d;
			o+=d*lo;
			ans+=minn*lo;
		}
		minn=min(minn,a);
	}
	cout<<ans;
	return 0;
}