#include <stdio.h>

int n, k;
int nc;
double p[26], pa[4000000];


void genereaza_combinari(int, int, double);
double calculeaza_probabilitate();

int main()
{
	freopen("dezastru.in", "r", stdin);
	freopen("dezastru.out", "w", stdout);
	
	scanf("%d %d", &n, &k);
	for (int i = 1; i <= n; ++i)
	{
		scanf("%lf", &p[i]);
	}

	genereaza_combinari(1, 0, 1.0f);
	double prob = calculeaza_probabilitate();
	
	printf("%.6lf", prob);
	return 0;
}

void genereaza_combinari(int curent, int start, double prob)
{
	if (curent <= k)
	{
		for (int i = start + 1; i <= n; ++i)
		{
			genereaza_combinari(curent + 1, i, prob * p[i]);
		}
	}
	else 
	{
		pa[++nc] = prob;
	}
}

double calculeaza_probabilitate()
{
	double p = 0.0;
	for (int i = 1; i <= nc; ++i)
	{
		p += (pa[i] / nc);
	}
	
	return p;
}