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#include <iostream>
#include <vector>

#define DEBUG 0

std::vector<int> a;

int main()
{
	int n, k, sum = 0;

	std::cin >> n >> k;
	a.resize(n);

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

	for (int i = 1; i < n; ++i) {
		int diff = a[i-1] - a[i];
		if (diff > k) {
			a[i] += diff - k;
			sum += diff - k;
		}
	}

	for (int i = n-2; i >= 0; --i) {
		int diff = a[i+1] - a[i];
		if (diff > k) {
			a[i] += diff - k;
			sum += diff - k;
		}
	}

#if DEBUG
	for (int i = 0; i < n; ++i){
		std::cout << a[i] << " ";
	}
	std::cout << std::endl;
#endif

	std::cout << sum << std::endl;

	return 0;
}