/*
TASK: seq
LANG: C++
*/
#include <iostream>

using namespace std;

#define _MAX 1000000

int n;
int k;
long long int r;
int a[_MAX];

void init()
{
	cin >> n;
	cin >> k;
	for (int i = 0 ; i < n; i++)
		cin >> a[i];
}

void solve()
{
	int count_numbers;
	long long int count_combinations;
	long long int factoriel = 1;
	long long int k_factoriel = 1;
	
	for (int i = 2; i <= k; i++)
		k_factoriel *= i;
	for (int i = n; i > n - k; i--)  // needs optimization
	{
		factoriel *= i;
	}
	count_combinations = factoriel / k_factoriel;
	count_numbers = (count_combinations * (n - k)) / n;
	for (int i = 0; i < n; i++)
		r += (a[i] * count_numbers);
	
	cout << r << endl;
}

int main()
{
	init();
	solve();
	
    return 0;
}
