#include <cstdio>
#include <algorithm>
#include <queue>

const int N = 2505;

int a[N], s[N];
double dp[N][N];

int main() {
    int n, m;
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; ++i) {
        scanf("%d", &a[i]);
    }
    std::sort(a + 1, a + n + 1, std::greater<int>());
    for (int i = 1; i <= n; ++i) {
        s[i] = s[i - 1] + a[i];
        dp[i][0] = s[i];
        dp[i][i] = s[i] / 2.0;
        for (int j = 1; j < i; ++j) {
            dp[i][j] = (a[i] - (dp[i - 1][j - 1] - dp[i - 1][j])) / 2 + dp[i - 1][j - 1];
            // printf("%d %d %6lf\n", i, j, dp[i][j]);
        }
    }
    printf("%.6lf\n", dp[n][m]);
}