#include<bits/stdc++.h>
using namespace std;
long long a[100001],b[100001];
int main()
{
	freopen("road.in","r",stdin);
	freopen("road.out","w",stdout);
	long long n,k,mi=1e9,s=0,l=0;
	cin>>n>>k;
	for(int i=1;i<n;i++)
	{
		cin>>a[i]; 
	}
	for(int i=1;i<n;i++)
	{
		cin>>b[i];
		mi=min(b[i],mi);
		if(l>=a[i])
		{
			l-=a[i];
			continue;
		}
		if((a[i]-l)%k==0) s+=(a[i]-l)/k*mi,l=0; 
		else
		{
			s+=((a[i]-l)/k+1)*mi;
			l=((a[i]-l)/k+1)*k+l-a[i];
		}
	}
	cin>>b[n];
	cout<<s<<endl;
	
	return 0;
}