#include<bits/stdc++.h>  
using namespace std;  
long long n,i,a[100005],v[100005];  
int main()  
{  
	int j=0,minn=INT_MAX,d;  
	cin>>n>>d;  
	for(i=1;i<=n-1;i++)  
    cin>>v[i];  
    for(i=1;i<=n;i++)  
    {  
    	cin>>a[i];  
    	if(a[i]<minn)  
    	minn=a[i];  
    	if((v[i]/d)*d<v[i])  
    	{  
    		j+=(v[i]/d)*minn+minn;  
			v[i+1]=v[i]+v[i+1]-(v[i]/d+1)*d;  
		}  
		else  
		{  
			j+=(v[i]/d)*minn;  
		}  
    }  
    cout<<j;   
}