#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
float a[100],s=1,sol;
int numi,k,n,fr[100],kk;
ifstream f("dezastru.in");
ofstream g("dezastru.out");

void Back (int k)
{
    int i,j;
    for (i=1;i<=n;i++)
    {
        fr[i]++;
        if (fr[i]==1)
        {
            s*=a[i];
        if (k<kk) {Back(k+1);} else
         {sol+=s; }
            s/=a[i];
        }

        fr[i]--;
    }
}


int main()
{
    int i;
f>>n>>kk;
numi=1;
for (i=1;i<=n;i++) {
    f>>a[i];
    numi*=i;
}
Back (1);

g<<setprecision(6)<<(double)sol/numi;


    return 0;
}