#include<bits/stdc++.h>
using namespace std;
int n,m,a[5],b[6],ans,minp,k,l,s;
int main(){
    cin>>n>>m;
    for(int i=0;i<n-1;i++) cin>>b[i+1];
    for(int i=0;i<n;i++) cin>>a[i];
    for(int i=0;i<n;i++){
		if(i==0) minp=a[i];
		else if(minp>a[i]) minp=a[i];
		s=b[i+1]-k;
		if(s%m!=0) l=s/m+1;
		else l=s/m;
		k+=l*m-b[i+1];
		ans+=l*minp;
	}
	cout<<ans;
    return 0;
}