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#include <fstream>
#include <vector>
#include <algorithm>
#include <iomanip>
using namespace std;
ifstream be("dezastru.in");
ofstream ki("dezastru.out");
int n,k;
float x[30];
double b[26];
long double db;
int main()
{
be>>n>>k;
for(int i=1;i<=n;i++)
be>>x[i];
vector<int>v;
for(int i=0;i<n;i++)
v.push_back(i+1);
b[-1]=1;
int m=0;
do{
for(int i=0;i<k;i++){
b[i]=b[i-1]*x[v[i]];
}
m++;
db+=b[k-1];
}while(next_permutation(v.begin(),v.end()));
ki<<setprecision(6)<<db/m<<endl;
return 0;
}