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

vector <long long> V;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    long long n;
    long long k, wyn=0;
    cin >> n >> k;
    for (int i=0; i<n; i++) {
        long long a;
        cin >> a;
        V.push_back(a);
    }
    vector <long long> P(n);
    vector <long long> S(n);
    P[0] = V[0];
    for (int i=1; i<n; i++) {
        P[i] = max(P[i-1], V[i]+(k*i));
    }
    S[n-1] = V[n-1] - k*(n-1);
    for (int i=n-2; i>=0; i--) {
        S[i] = max(S[i+1], V[i]-(k*i));
    }
    for (int i=0; i<n; i++) {
        wyn += max(P[i]-(k*i), S[i]+(k*i)) - V[i];
    }
    cout << wyn;
    return 0;
}