#include <fstream>
using namespace std;

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

int n, k;
double p[27], a[27][27];

int com(int n, int k)
{
    if (n == k || k == 0) {
        return 1;
    }
    return com(n - 1, k) + com(n - 1, k - 1);
}

int main()
{
    fin>>n>>k;
    for(int i=0;i<=n;++i) a[i][0]=1;
    for(int i=1;i<=n;++i)
        fin>>p[i];
    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];
    fout<<a[n][k]/com(n, k)<<"\n";
    return 0;
}