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

#define MAXN 27

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

int c[MAXN][MAXN],n,k,i,j;
float d[MAXN][MAXN],prob[MAXN];


int main()
{
    fin>>n>>k;
    for(i=1;i<=n;i++) {
        fin>>prob[i];
        d[0][i]=0;
        d[i][0]=1;
    }
    c[0][0]=1;
    d[0][0]=1;
    d[1][1]=prob[1];

    for(i=1;i<=n+1;i++) {
        for(j=1;j<=n+1;j++) {
            if (i>=j)
            d[i][j]=d[i-1][j]+d[i-1][j-1]*prob[i];
            c[i][j]=c[i-1][j-1]+c[i-1][j];
        }
    }
    fout<<d[n][k]/c[n+1][k];
    fout.close();
    return 0;
}