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#include <iostream>
#include <vector>
#include <algorithm>
bool Update(std::vector<int>& a, long long& result, int k) {
	std::vector<int> temp(a);
	for (int i = 0; i < temp.size()-1; i++) {
		if ((temp[i + 1] - temp[i] > k)) {
			result += temp[i + 1] - (temp[i]+k);
			temp[i] = temp[i + 1] - k;
		}
	}
	for (int i = a.size() - 1; i > 0; i--) {
		if ((temp[i - 1] - temp[i] > k)) {
			result += temp[i - 1] - (temp[i] + k);
			temp[i] = temp[i - 1] - k;
		}
	}
	if (temp == a) {
		a = temp;
		return true;
	}
	a = temp;
	return false;
}
int main() {
	short n;
	int k;
	long long result = 0;
	std::cin >> n >> k;
	std::vector<int> a(n);
	std::cin >> a[0];
	for (int i = 1; i < n; i++) {
		std::cin >> a[i];
	}
	while (!Update(a, result, k));
	std::cout << result;
}