#include <iostream>
#include <vector>

using namespace std;

int main() {
	int N, K;
	cin >> N >> K;

	vector<int> B(N); // Bájtra

	for (int i = 0; i < N; i++) cin >> B[i];

	int mp = INT_MAX;
	for (int i = 0, mi, s; i < N - K; i++) {
		mi = 0;
		s = 0;
		for (int j = i; j < i + K; j++) {
			s += B[j];
			if (B[j] >= B[mi]) mi = j;
		}

		/*for (int j = i; j < i + K; j++) {
			if (j < mi) sl += B[j];
			else if (j > mi) sr += B[j];
		}*/
		//int p = ((B[mi] + mi - i) + (B[mi] +1)) * (mi-i) / 2;
		//p += ((B[mi]-i-K+mi)+(B[mi]-1))*(i+K-mi)/2;
		int p = ((B[mi] + mi - i) + (B[mi] + mi - i - K + 1)) * K / 2;
		p -= s;
		if (p < mp) mp = p;
	}

	cout << mp << endl;
	return 0;
}