#include <iostream>
#include <fstream>
#include <iomanip>
#define NMax 27
using namespace std;
float V[NMax];
float A[NMax][NMax];
int N,k;

int main()
{
    ifstream g("dezastru.in");
    g>>N>>k;
    for(int i=1;i<=N;i++)
        g>>V[i];
    for(int i=0;i<=N;i++)
        A[i][0]=1;

    for(int i=1;i<=N;i++)
    {
        int i1=min(k,i);
        for(int j=1;j<=i1;j++)
            A[i][j]=A[i-1][j-1]*V[i]+A[i-1][j];
    }

    float a1=1,c1=1;
    for(int i=N-k+1;i<=N;i++)
        a1=a1*i;
    for(int i=1;i<=k;i++)
        c1=c1*i;
        c1=a1/c1;
    ofstream f("dezastru.out");
    f<<fixed<<setprecision(6)<<A[N][k]/c1;
}