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#include <iostream>
#include <vector>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    long long res{};
    int n, k;
    cin >> n >> k;
    vector<int> path;
    path.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> path[i];
    }
    for (int i = 0; i < n - 1; ++i) {
        auto diff = path[i] - path[i + 1] - k;
        if (diff > 0) {
            res += diff;
            path[i + 1] += diff;
        }
    }
    for (int i = n - 2; i >= 0; --i) {
        auto diff = path[i + 1] - path[i] - k;
        if (diff > 0) {
            res += diff;
            path[i] += diff;
        }
    }

    cout << res << "\n";
}