#include <iomanip>
#include <fstream>

using namespace std;

ifstream fi("dezastru.in");
ofstream fo("dezastru.out");

double sol=0;
double v[30];
int n,m;
int st[30];
int nrf = 0;

inline void bkt(int k, int s, double sum)
{
    if (k>m)
    {
        sol+=sum;
        nrf++;
        return;
    }
    for (register int i=s+1;i<=n-m+k;i++)
        bkt(k+1,i,sum*v[i]);
}
int main()
{
    fi>>n>>m;
    for(int i=1;i<=n;i++)
        fi>>v[i];
    bkt(1, 0, 1);
    fo<<setprecision(6)<<fixed<<(double)sol/nrf;
    return 0;
}