#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=1e5+15;
int n,d,ans,o,a[N],v[N],sum[N],r;
signed main(){
	cin>>n>>d;
	int minx=1e5+10;
	for(int i=1;i<=n-1;i++){
		cin>>v[i];
	}
	for(int i=1;i<=n;i++){
		cin>>a[i];
	}
	for(int i=1;i<=n-1;i++){
		minx=min(minx,a[i]);
		if(r>=v[i]){
			r-=v[i];
			continue;
		}
		r=v[i]-r;
		o=ceil(1.0*r/d);
		r=o*d-v[i];
		ans+=r*minx;
	}
	cout<<ans; 
	return 0;
}