#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;
}