#include<bits/stdc++.h>
using namespace std;	
const int N=1e5+10;
long long n,d;
long long v[N],a[N];
int main(){
	cin>>n>>d;
	for(int i=1;i<n;i++) cin>>v[i];
	for(int i=1;i<=n;i++) cin>>a[i];
	long long L=0,dy,s=0,minn=a[1];
	//L是所需油量,dy是多余的距离,s是花的钱,minn是目前最低油价 
	for(int i=1;i<n;i++){
		L=ceil((v[i]-dy)*1.0/d);
		dy+=L*d-v[i];
		minn=min(minn,a[i]);
		s+=L*minn; 
		//cout<<L<<" "<<dy<<" "<<s<<" "<<minn<<endl; 
	}
	cout<<s;
	return 0;
}