#include <bits/stdc++.h>

using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
double v[26],s=0;
int uz[26],x[26],n,k;
 void sum()
{
    double p=1;
    int i;
    for(i=1;i<=k;i++) p=p*v[x[i]];
    s=s+(p/(n*k));
}
void back(int k)
{
    int val;
    if(k==n+1) sum();
    else for(val=1;val<=n;val++)
    if(uz[val]==0) {x[k]=val;
                   uz[val]=1;
                   back(k+1);
                   uz[val]=0; }
}
int main()
{int i,j;
f>>n>>k;
for(i=1;i<=n;i++)
    f>>v[i];
back(1);
g<<setprecision(6)<<s;
}