#include<bits/stdc++.h>
using namespace std;
#define int long long 
const int N=1e5+10;
int p[N],v[N];
int n,d,ans=0;
signed main(){
//	freopen("road.in","r",stdin);
//	freopen("road.out","w",stdout);
	cin>>n>>d;
	for(int i=1;i<n;i++){
		cin>>v[i];
	}
	for(int i=1;i<=n;i++){
		cin>>p[i];
		if(i==1) continue;
		p[i]=min(p[i],p[i-1]);
	}
	int sy=0;
	for(int i=1;i<n;i++){
		if(sy>=v[i]) sy-=v[i];
		else{
			int jl=v[i]-sy;
			int s=ceil(jl*1.0/d);
			ans+=s*p[i];
			sy=s*d-jl;
		}
	}
	cout<<ans;
	return 0;
}