#include<cstdio>

using namespace std;

#define Nmax 32

int N, M, nrperm;
double A[Nmax], sum;

void back(int k, int poz, double prod) {
	if(k==M) {
		nrperm++;
		sum+=prod;
		return;
	}
	int i;
	for(i=poz+1; i<=N-M+k; i++)
		back(k+1, i, prod*A[i]);
}

int main() {
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);

	int i;
	scanf("%d %d",&N,&M);
	for(i=0; i<N; i++)
		scanf("%lf",A+i);

	back(0,-1,1);

	printf("%.6lf\n", sum / nrperm);
	
	return 0;
}