#include<bits/stdc++.h>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int u[30],n,k,K,nr;
double v[30],P;
void back(int k, double p, int j)
{
    if(k>K)
    {
        P+=p;
        nr++;
        return ;
    }
    for(int i=j+1;i<=n;i++)
        if(u[i]==0)
    {
        u[i]=1;
        back(k+1,p*v[i],i);
        u[i]=0;
    }
}
int main()
{
    f>>n>>K;
    for(int i=1;i<=n;i++)
        f>>v[i];
    back(1,1,0);
    g<<setprecision(6)<<fixed<<P/nr;
}