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

int const K = 1000001;
int tab[K];
int temp[K];
int k;
long n, res;
int main()
{
	cin >> n >> k;
	for (int i = 0; i < n; i++)
	{
		cin >> tab[i];
	}

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

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

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

    long long res = 0;

    for (int i = 0; i < n; i++)
    {
        result[i] = max(left[i], right[i]);
        res += (long long)(result[i] - tab[i]);
    }
	cout << res;
	return 0;
}