using namespace std;
#include<fstream>
#include<algorithm>
ifstream eu("dezastru.in");
ofstream tu("dezastru.out");
#define Nmax 100
int N,K;
double v[Nmax],s,nr;
 
void back(int l,int x,double p)
{
    int i;
	if(l==K+1)
	{
        s+=p;
		++nr;
	}
    else
        for(i=x;i<=N-K+l;++i)
            back(l+1,i+1,p*v[i]);
}
int main()
{
	eu>>N>>K;
	for(int i=1;i<=N;i++)
		eu>>v[i];
    back(1,1,1);
	tu<<s/nr;
    return 0;
}