#include <cstdio>
#include <cstdlib>

using namespace std;

#define NMAX 32

int v[NMAX];
float P[NMAX];
int N, K;

double s = 0;
long long n = 0;

bool b[NMAX];

void print(){
	for(int i = 1; i < K; ++i)
		for(int j = i+1; j <= K; ++j)
			if(v[i] == v[j])
				return;

	double p  = 1;
	for(int i = 1; i <= K; ++i)
			//printf("%d ", v[i]);
			p *= P[v[i]];
	s += p;
	++n;
}

void back(int i){	
	if(i > K){
		print();
		return;
	}

	for(int j = 1; j <= N; ++j)
		if(b[j] == false){
			v[i] = j;
			b[j] = true;
			back(i+1);
			b[j] = false;
		}
}	

int main(){
	freopen( "dezastru.in",  "rt",  stdin );
	freopen("dezastru.out",  "wt", stdout  );

	scanf("%d %d", &N, &K);
	
	for(int i = 1; i <= N; ++i)
		scanf("%f", &P[i]);
		
	back(1);
	
	printf("%.6f", s/n);
	
	return 0;
}