#include<bits/stdc++.h>
using namespace std;
#define int long long
int dd,ans;
int a[1000009];
int v[1000009];
signed main(){
	int n,d;
	cin>>n>>d;
	for(int i=1;i<n;i++){
		cin>>v[i];
	}
	for(int i=1;i<=n;i++){
		cin>>a[i];
	}
	a[n]=-1;
	int t;
	t=1;
	while(1){
		if(t==n){
			cout<<ans<<endl;
			return 0;
		}
		int sum;
		sum=0;
		for(int i=t+1;i<=n;i++){
			sum+=v[i-1];
			if(a[i]<a[t]){
				sum-=dd;
				if(sum%d){
					ans+=(sum/d+1)*a[t];
					dd=(sum/d+1)*d-sum;
				}else{
					ans+=sum/d*a[t];
					dd=0;
				}
				t=i;
				break;
			}
		}
	}
	return 0;
}