#include <stdio.h>
#include <fstream>


using namespace std;

int n,k,m,st[26],contor = 0;
double a[26],s = 0;

void solve()
{
    double p = 1;
    contor++;
    for(int i=1; i<=m; i++)
     p *= a[st[i]];
    s += p;
}

void back(int k)
{
    if(k == n)
      solve();
    else
     for(int i=st[k-1]+1; i<=n-m+k; i++)
       {
           st[k] = i;
           back(k+1);
       }
}

int main()
{
    FILE *f= fopen("dezastru.in","r");
    fscanf(f,"%d %d", &n,&m);
    for(int i=1; i<=n; i++)
       fscanf(f,"%lf", &a[i]);

    st[0] = 0;
    back(1);

    f = fopen("dezastru.out","w");
    fprintf(f,"%0.6lf", s/contor);

    return 0;
}