#include <bits/stdc++.h>

using namespace std;
ifstream in("dezastru.in");
ofstream out("dezastru.out");
int x[30],cnt,k,nr,n;
double v[30],prod,sol=0;
void back(int k)
{
    if(k<=cnt)
    {
        for(int i=x[k-1]+1; i<=n-(cnt-k+1)+1; i++)
        {
            x[k]=i;
            prod*=v[i];
            back(k+1);
            prod/=v[i];
        }
    }
    else
    {
        sol+=prod;
        nr++;
    }
}
int main()
{
    in>>n>>cnt;
    for(int i=1; i<=n; i++)
        in>>v[i];
    prod=1;
    back(1);
    out<<fixed<<setprecision(6)<<(double)sol/nr;
    return 0;
}