#include <iostream>
#define N 100010
#define ll long long
using namespace std;
int n;
ll d, v[N], p[N], ans;
int main() {
    cin >> n >> d;
    for (int i = 1; i < n; i++) cin >> v[i];
    for (int i = 1; i <= n; i++) cin >> p[i];
    int now = 1, nxt = 2;
    ll tank = 0;
    while (now < n) {
        ll dis = v[now];
        while (nxt < n && p[nxt] >= p[now]) 
            dis += v[nxt++];
        int cnt = 0;
        while (tank + cnt * d < dis) cnt++;
        ans += p[now] * cnt;
        tank += cnt * d - dis;
        now = nxt++;
    }
    cout << ans << endl;
    return 0;
}