#include<bits/stdc++.h>
#define int long long
using namespace std;
int n, a[100010], v[100010], d, ans; 
signed 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 minn = a[1];
	for(int i = 1;i <= n - 1;i++){
		int w = v[i] / d + 1;
		//cout << w << endl;
		if(v[i] % d == 0) w--;
		if(a[i] < minn) minn = a[i];
		ans += w * minn;
		//cout << minn << " " << w << endl;
		v[i + 1] -= (w * d - v[i]);
	}
	cout << ans;
}

感觉没啥错，考试时大样例也过了，但不对