#include <fstream>
#include <iomanip>

using namespace std;

const int Nmax=30;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

double Pr[Nmax], Dp[Nmax][Nmax];
int Comb[Nmax][Nmax];

int main()
{
    int N, K;
    fin>>N>>K;

    for(int i=1;i<=N;i++) fin>>Pr[i];

    Dp[0][0]=Comb[0][0]=1;
    for(int i=1;i<=N;i++)
    {
        Dp[i][0]=Comb[i][0]=1;
        for(int j=1;j<=K;j++)
        {
            Dp[i][j]=Dp[i-1][j]+Dp[i-1][j-1]*Pr[i];
            Comb[i][j]=Comb[i-1][j]+Comb[i-1][j-1];
        }
    }

    fout<<setprecision(6)<<fixed;
    fout<<Dp[N][K]/Comb[N][K]<<"\n";

    fin.close();
    fout.close();
}