#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
#define MAX 27
#define f (long double)

long double p[MAX], a[MAX][MAX];
int c[MAX][MAX];

int main()
{
    int n, k, i, j;
    fin>>n>>k;
    for(i=1;i<=n;i++)
    {
        fin>>p[i];
    }
    for(i=0;i<=n;i++)
        a[i][0]=1;
    c[0][0]=1;
    for(i=1;i<=n+1;i++)
    {
        for(j=1;j<=k+1;j++)
        {
            a[i][j]=f a[i-1][j]+f a[i-1][j-1]*p[i];
            c[i][j]=c[i-1][j]+c[i-1][j-1];
        }
    }

    fout<<a[n][k]/c[n+1][k+1];
}