#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
    
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    long long k;
    if (!(cin >> n >> k)) return 0;


    vector<long long> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }

    
    vector<long long> L(n);
    L[0] = a[0];
    for (int i = 1; i < n; ++i) {
        L[i] = max(a[i], L[i - 1] - k);
    }

    
    vector<long long> R(n);
    R[n - 1] = a[n - 1];
    for (int i = n - 2; i >= 0; --i) {
        R[i] = max(a[i], R[i + 1] - k);
    }

   
    long long total_gravel = 0;
    for (int i = 0; i < n; ++i) {
        long long final_height = max(L[i], R[i]);
        total_gravel += final_height - a[i];
    }

    
    cout << total_gravel << "\n";

    return 0;
}