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

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

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

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

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

    cout << total_gravel << endl;

    return 0;
}