#include<bits/stdc++.h>
#define ll long long 
using namespace std;
ll n,d,v[100005],a[100005],s=0;
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];
	}
	for(int i=1;i<=n-1;i++){
		ll m=v[i];
		if(a[i]<a[i+1]){
			for(int j=1;j<=n-i;j++){
				if(a[i]<a[i+j]) m+=v[i+j];
			}
			s+=ceil(m/d*1.0)*a[i];
		}
		else s+=ceil(v[i]/d*1.0)*a[i];
	}
	cout<<s;
	return 0;
} 

谢谢大佬们！