#include<bits/stdc++.h>
using namespace std;
int t[1000000], a[1000000], n, d;
int main() {
	cin>>n>>d;
	for (int i=1; i<n; i++) cin>>t[i];
	int minn=1000000, ans=0, s=0;
	for (int i=1; i<n; i++) {
		cin>>a[i];
		s+=t[i];
		minn=min(minn, a[i]);
		if (s>0) {
			ans+=((d+s-1)/d)*minn;
			s-=((d+s-1)/d)*d;
		}
	}
	cout<<ans;
	return 0;
}