#include<bits/stdc++.h>
using namespace std;
const int N=1e5+7;
int arr[N],prize[N];
int main(){
//	freopen("road.in","r",stdin);
//	freopen("road.out","w",stdout);
	long long sum=0,tmp=0;
	int n,k;
	cin>>n>>k;
	for(int i=1;i<n;i++){
		cin>>arr[i];
		arr[i]+=arr[i-1];
	}
	for(int i=1;i<=n;i++) cin>>prize[i];
	for(int i=1;i<n;i++){
		if(prize[i+1]<prize[i]){
			long long num=(arr[i]-arr[i-1]-1-tmp)/k+1;
			sum+=(num*prize[i]);
			tmp+=(num*k);
			tmp-=(arr[i]-arr[i-1]);
		}else{
			if(i==n-1){
				long long num=(arr[i]-arr[i-1]-1)/k+1;
				sum+=(num*prize[i]);
				break;
			}
			int id=i;
			while(id<n){
				id++;
				if(prize[i]>=prize[id]) break;
			}
			long long num=(arr[id-1]-arr[i-1]-1-tmp)/k+1;
			tmp+=(num*k);
			tmp-=(arr[id-1]-arr[i-1]);
			sum+=(num*prize[i]);
			i=id-1;
		}
	}
	cout<<sum;
	return 0;
}