#include <bits/stdc++.h>

using namespace std;

const int N = 1e5+10;

int n, d, v[N], a[N];

int ceild(int a, int b) {
	int ans=a/b;
	ans+=(a%b==0)?0:1;
	return ans;
}

int main() {
	cin >> n >> d;
	for(int i = 1; i <= n-1; i++) cin >> v[i];
	for(int i = 1; i <= n; i++) cin >> a[i];
	
	int cheap = a[1],ans=0,road=0;
	for(int i = 1; i <= n-1; i++) {
		//从第i开到第i 
		cheap = min(a[i], cheap);
		int need = ceild(v[i]-road, d);
		ans+=need*cheap;
		//计算开到第i+1站耗费多少油
		road=need*d-v[i];
	}
	cout<<ans;
	return 0;
}