#include <stdio.h>

int N, K;
double prob[26];
double S = 0;
long long  fact = 0;

void back (int k, double x, int last)
{
	if (k == K + 1)
	{
		S += x;
		fact ++;
	} 
	else
	{
		for (register int i = last + 1; i <= N - K + k; ++i)
			back(k + 1, x * prob[i], i);
	}
}

int main()
{
	FILE *f = fopen ("dezastru.in", "r");
	FILE *g = fopen ("dezastru.out", "w");
	
	fscanf (f,"%d %d", &N, &K);
	
	for (register int i=1; i<=N; ++i) 
		fscanf (f,"%lf", &prob[i]);
	
	back(1, 1, 0);
	
	fprintf (g,"%lf\n", S / fact);
	
	fclose(g);
	fclose(f);
	return 0;
}