#include <stdio.h>

int st[26];
double prob[26], s, p;
int n, K;
int k, AS;
long int fact;

double back (int k)
{
	if (k == K + 1)
	{
		/*for (i=1; i<=K; ++i)
			printf ("%d ", st[i]);
		printf ("\n");*/
		fact ++;
		p = 1;
		for (register int i=1; i<=K; ++i)
			p *= prob[st[i]];
		s += p;
	}
	else
	{
		if (k == 1)
			st[k] = 0;
		else
			st[k] = st[k-1];
		while (st[k] < n)
		{
			st[k] ++;
			back (k + 1);
		}
	}
	return s;
}

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]);
	
	fprintf (g,"%6lf", back (1)/fact);
	fclose(g);
	fclose(f);
	return 0;
}