#include <bits/stdc++.h>
#include <cstdio>
using namespace std;
#define ll long long
ll n,ddh;
ll d[100009];
ll a[100009]; 
int main(){
	cin>>n>>ddh;
	for(ll i=1;i<n;i++){
		cin>>d[i];
	}
	for(ll i=1;i<=n;i++){
		cin>>a[i];
	}
	ll sum=0;
	ll st=a[1];
	ll ans=0; 
	for(ll i=1;i<n;i++){
		if(st>a[i]){
			ll x=sum%ddh;
			ans+=ceil(sum*1.0/ddh*1.0)*st;
			st=a[i];
			sum=-1*(ddh-x);
			sum+=d[i];
		}
		else{
			sum+=d[i];
		}
	}
	ans+=ceil(sum*1.0/ddh*1.0)*st;
	cout<<ans;
	return 0;
}