#include <iostream>
#include <vector>
#include <limits.h>

using namespace std;

int main() {
	#define int long long
	#define INT_MAX LLONG_MAX
	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 = 0, s = 0; i < N - K; i++) {
		
		if (i == 0) {
			for (int j = i; j < i + K; j++) {
				s += B[j];
				if (B[j] >= B[mi]) mi = j;
			}
		}else{
			s += B[i + K - 1];
			s -= B[i - 1];
			if (B[i + K - 1] >= B[mi]) mi = i + K - 1;
			if (mi < i) {
				mi = i;
				for (int j = i; j < i + K; j++) if (B[j] >= B[mi]) mi = j;
			}
		}

		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;
}