#include <fstream>

using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
const int NMAX=25;
double dp[NMAX+5][NMAX+5];
double v[NMAX+5];
double comb(double n, double k)
{
    if(k==1)
        return n;
    else
      return 1ll*(n-k+1)*comb(n, k-1)/k;
}
int main()
{
    int i, n, k, j;
    double x;
    fin>>n>>k;
    dp[0][0]=1;
    for(i=1;i<=n;i++)
    {
        fin>>x;
        v[i]=x;
        dp[i][0]=1;
    }
    for(i=1;i<=n;i++)
    {
        for(j=1;j<=i;j++)
            dp[i][j]=dp[i-1][j-1]*v[i]+dp[i-1][j];
    }
    double sol;
      fout<<dp[n][k]/comb(n*1.0, k*1.0)<<"\n";
    return 0;
}