#include <fstream>
#include <iomanip>
#include <math.h>
#define Nmax 30
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");

int N,K,st[Nmax],used[Nmax],NrPerm,C[Nmax][Nmax];
double P[Nmax],A[Nmax][Nmax],S=0.0;


int main()
{

    f>>N>>K;
    A[0][0]=C[0][0]=1;
    for(int i=1;i<=N;++i)f>>P[i],C[i][0]=A[i][0]=1;
    for(int i=1;i<=N;++i)
        for(int j=1;j<=K;++j)C[i][j]=C[i-1][j]+C[i-1][j-1];
    for(int i=1;i<=N;++i)
        for(int j=1;j<=K;++j)A[i][j]=A[i-1][j]+A[i-1][j-1]*P[i];
    g<<fixed<<setprecision(6)<<fabs(A[N][K]/C[N][K])<<'\n';
    f.close();g.close();
    return 0;
}