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

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int n, k;
    int sum = 0;
    cin >> n >> k;
    vector<int> path(n);
    for (int i = 0; i < n; i++)
    {
        cin >> path[i];
    }
    for (int i = 0; i + 1 < n; i++)
    {
        if (path[i] - path[i + 1] > k)
        {
            sum += path[i] - path[i + 1] - k;
            path[i + 1] = path[i] - k;
        }
    }
    for (int i = n - 1; i - 1 >= 0; i--)
    {
        if (path[i] - path[i - 1] > k)
        {
            sum += path[i] - path[i - 1] - k;
            path[i - 1] = path[i] - k;
        }
    }
    cout << sum << "\n";
}