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#include <iostream>
#include <vector>

using namespace std;

int main()
{
    int n, k;

    cin >> n >> k;

    vector<int> f(n);

    for (int i = 0; i < n; i++)
    {
        cin >> f[i];
    }

    vector<int> left(n), right(n);

    left[0] = f[0];
    for (int i = 1; i < n; i++)
    {
        left[i] = max(f[i], left[i - 1] - k);
    }

    right[n - 1] = f[n - 1];
    for (int i = n - 2; i >= 0; i--)
    {
        right[i] = max(f[i], right[i + 1] - k);
    }

    int result = 0;
    for (int i = 0; i < n; i++)
    {
        result += max(left[i], right[i]) - f[i];
    }

    cout << result;
}