#include<iostream>
using namespace std;

const int N = 1e5 + 10;
typedef long long LL;   //经计算，总公里数、总油耗、总油费会爆Int
int v[N], a[N];     //分别对应站点距离和站点油价
int n, d;   //站点数和每升油的前进距离
int price;  //此站点前面可选的最低油价

int main(){
    cin >> n >> d;
    for(int i = 1; i < n; i++)
        cin >> v[i];
    for(int i = 1; i <= n; i++)
        cin >> a[i];

    LL dist = 0, oil = 0, ans = 0;  //  分别对应到当前站点的总距离，已加的油量和总油费
    //采用赊的方式，从第二个站点开始
    price = a[1];   //对于第二个站点，别无选择，只能选第一个的油价

    for(int i = 2; i <= n; i++){    //这里i要取到等号
        dist += v[i - 1];
        //(dist + d - 1) / d 是将对dist / d向上取整转化为向下取整
        ans += ((dist + d - 1) / d - oil) * price;
        price = min(price, a[i]);
        oil += (dist + d - 1) / d - oil;
    }

    cout << ans << endl;    

    return 0;
}