#include <iostream>
#include <fstream>
using namespace std;

#define MAXN 26

typedef long double real64;

real64 a[MAXN][MAXN], v[MAXN];
int c[MAXN][MAXN];

int main ()
{
    fstream fin ("dezastru.in", ios::in);
    fstream fout ("dezastru.out", ios::out);

    int n, k;

    fin >> n >> k;

    a[0][0] = 1;
    for (int i = 1; i <= n; ++i) {
        fin >> v[i];
        a[i][0] = 1;
        c[i][i] = c[i][0] = 1;
    }

    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= i; ++j) {
            a[i][j] = a[i - 1][j] + v[i] * a[i - 1][j - 1];

            c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
        };
        c[i][i] = 1;
    }

    fout << a[n][k] / c[n][k] << endl;

    fin.close ();
    fout.close ();
    return 0;
}
          }
        
        
          int main()
          {int i,j;
              f>>n>>k;
              
          
         for(i=1;i<=n;i++)
        f>>a[i];
         
   for(i=k+1;i<=n;i++)
     fact_k=fact_k*i;

//pt fiecare combinare k elem   k! posibilitati de atac
//      soltuia finala=   posibiliati/(k+1)*(k+2)*..n
// ar fi trebuit   posibilitati*k!/n!

     
          back(1);
          g<<setiosflags(ios::showpoint)<<setprecision(6)<<(double)total_probabil/fact_k;
          f.close();
          g.close();
      
              return 0;
          }