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#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;
    cin >> n >> k;

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

    // Przejście w prawo
    for (int i = 1; i < n; i++) {
        a[i] = max(a[i], a[i-1] - k);
    }

    // Przejście w lewo
    for (int i = n - 2; i >= 0; i--) {
        a[i] = max(a[i], a[i+1] - k);
    }

    long long total_gravel = 0;
    for (int i = 0; i < n; i++) {
        total_gravel += (a[i] - original[i]);
    }

    cout << total_gravel << endl;

    return 0;
}