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#include <iostream>
#include <vector>

using namespace std;
typedef long long LL;
typedef vector<LL> VI;

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

	int n; LL k;
	cin >> n >> k;

	VI a(n + 1);
	for (int i = 1; i <= n; ++i) cin >> a[i];
	VI leftMax(n + 1), rightMax(n + 1);

	leftMax[1] = a[1] + k * 1LL;
	for (int i = 2; i <= n; ++i) leftMax[i] = max(leftMax[i - 1], a[i] + k * 1LL * i);
	rightMax[n] = a[n] - k * 1LL * n;
	for (int i = n - 1; i >= 1; --i) rightMax[i] = max(rightMax[i + 1], a[i] - k * 1LL * i);

	LL ans = 0;
	for (int i = 1; i <= n; ++i)
	{
		LL bi = max(leftMax[i] - k * 1LL * i, rightMax[i] + k * 1LL * i);
		ans += bi - a[i];
	}
	cout << ans << endl;

	return 0;
}