#include <iostream>
#include <vector>

long smax(std::vector<long>& e, int K) {
	long maximum = 0;
	long sum = 0;
	int index = -1;
	for (size_t i = 0; i < K; ++i) {
		if (e[i] >= maximum) {
			maximum = e[i];
			index = i;
		}
	}
	for (size_t i = 0; i < K; ++i) {
		if (i < index || i > index) {
			if (e[i] <= maximum + (index - i)) {
				//std::cout << "1 " << maximum + (index - i) - e[i] << '\n';
				sum += maximum + (index - i) - e[i];
			}
			if (e[i] > maximum + (index - i)) {
				sum = sum + i;
				//std::cout << "2 " << sum << '\n';
				maximum += 1;
			}
		}
	}
	return sum;
}
int main() {
	std::vector<long> hegy{};
	std::vector<long> temp{};
	std::vector<long> koltsegek{};
	hegy.reserve(10000);
	temp.reserve(3);
	long H;
	int N;
	int K;
	std::cin >> N >> K;
	for (size_t i = 0; i < N; ++i) {
		std::cin >> H;
		hegy.emplace_back(H);
	}
	for (size_t i = 0; i < N - 2; ++i) {
		long koltseg = 0;
		temp.clear();
		temp.emplace_back(hegy[i]);
		temp.emplace_back(hegy[i + 1]);
		temp.emplace_back(hegy[i + 2]);
		koltsegek.emplace_back(smax(temp, K));
	}
	long ans = 100000000000000000;
	for (size_t i = 0; i < koltsegek.size(); ++i) {
		//std::cout << koltsegek[i] << '\n';
		if (koltsegek[i] < ans) {
			ans = koltsegek[i];
		}
	}
	std::cout << ans;
	return 0;
}