#include <bits/stdc++.h>

using namespace std;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n, k;
    cin >> n >> k;
    
    int tab [n];
    for(int i=0;i<n;i++) {
        cin >> tab[i];
    }
    
    int scr = 0, max_req;
    for(int i=0;i<n;i++) {
        max_req = 0;
        for(int j=0;j<n;j++) {
            if(tab[j] > tab[i] + abs(i-j)*k) {
                max_req = max(max_req, tab[j] - tab[i] - abs(i-j)*k);
            }
        }
        scr += max_req;
    }

    cout << scr << endl;
    return 0;
}