// Author : Jakub Rożek
// Task   : DOS - Dostawa żwiru [C]
// Memory : n
// Time   : n
// Solv   : standardowe - dwa przebiegi

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

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n;
    long long k, x, answer = 0;
    
    cin >> n >> k;

    vector<long long> a(n);
    for (long long &i : a) {
        cin >> i;
    }

    for (int i = 1; i < n; ++i) {
        x = a[i - 1] - a[i] - k;
        if (x > 0) {
            answer += x;
            a[i] += x;
        }
    }

    for (int i = n - 2; i >= 0; --i) {
        x = a[i + 1] - a[i] - k;
        if (x > 0) {
            answer += x;
            a[i] += x;
        }
    }

    cout << answer << '\n';
}