#include<fstream>
#define dim 28
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
double v[dim],s;
int n,c,nr;
inline void back(int k,int x,double p){
    if(k==c){
        s+=p;
        nr++;
        return;
    }
    for(int i=x;i<=n-c+k;++i)
        back(k+1,i+1,p*v[i]);
}
int main (){
    f>>n>>c;
    for( int i=0;i<n;i++)
        f>>v[i];
    back(0,0,1);
    g<<s/nr;
    return 0;
}