题解

#include<bits/stdc++.h>
typedef long long ll;
using namespace std;

const int N = 1e5 + 10;
int n,d,v[N],a[N],mn = 1e9;
ll ans,pre,sum;

int main(){
	freopen("road.in","r",stdin);
	freopen("road.out","w",stdout);
	cin>> n >> d;
    for(int i = 2;i <= n;i++) cin >> v[i];
    for(int i = 1;i <= n;i++) cin >> a[i];
    for(int i = 2;i <= n;i++){
        sum += v[i];//累加
        mn = min(mn,a[i-1]);//求最小油价
        ll num = max(0LL,(sum - pre + d - 1)/d);//需要购买的油量
        ans+=num*mn;//累加花费
        pre+=num*d;//到达下一个位置的距离
    }
    cout << ans;
	fclose(stdin);
	fclose(stdout);
	return 0;
}