import java.util.*;
import java.io.*;

public class travis
{
    public static void main(String[] Args) throws Exception
    {
        FS sc = new FS(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        Long[] arr = new Long[n];
        Long[] arr2 = new Long[n];
        for (int i = 0; i < n; i++) 
        {
            arr[i] = sc.nextLong();
        }
        Arrays.sort(arr);
        for (int i = 0; i < n; i++) 
        {
            arr2[i] = arr[i] * arr[i];
        }

        long sum = 0;
        long sum2 = 0;
        for (int i = 0; i < k; i++)
        {
            sum += arr[i];
            sum2 += arr2[i];
        }

        // default the answer to something really big
        double ans = 1000000000;
        ans *= ans;
        
        for (int i = 0; i + k <= n; i++)
        {
            double mu = sum;
            mu /= k;
            
            double tans = sum2 - (2 * mu * sum) + (k * mu * mu);

            if (tans < ans) ans = tans;

            if (i + k != n)
            {
                sum -= arr[i];
                sum2 -= arr2[i];
                sum += arr[i+k];
                sum2 += arr2[i+k];
            }
        }

        System.out.println(ans);
        
    }

    public static class FS 
    {
        BufferedReader br;
        StringTokenizer st;

        FS(InputStream in) throws Exception
        {
            br = new BufferedReader(new InputStreamReader(in));
            st = new StringTokenizer(br.readLine());
        }

        String next() throws Exception
        {
            if (st.hasMoreTokens()) return st.nextToken();
            st = new StringTokenizer(br.readLine());
            return next();
        }

        int nextInt() throws Exception
        {
            return Integer.parseInt(next());
        }

        long nextLong() throws Exception
        {
            return Long.parseLong(next());
        }
    }
}