#include <fstream>
#include <algorithm>
using namespace std;
int x[100], n, m;
double prob[40], probp[11414], probfin=0;
int contor=0;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int perm[55];
int main()
{
	f>>n>>m;
	int i;
	for(i=1; i<=n; i++)
	{
		f>>prob[i];
	}
	for(i=1; i<=n; i++)
	{
		perm[i]=i;
	}
	do
	{
		probp[contor]=1;
		for(i=1; i<=m; i++)
		{
			probp[contor]*=prob[perm[i]];
		}
		contor++;
	}while(next_permutation(perm+1, perm+n+1));
	for(i=0; i<contor; i++)
	{
		probp[i]/=contor;
		probfin+=probp[i];
	}
	g.precision(6);
	g<<fixed<<probfin;
}