#include <stdio.h>
#define fin  "dezastru.in"
#define fout "dezastru.out"
#define Nmax 26

double v[Nmax],ret;
int N,K;

void go(int lv,int last,double tmp) {
int i;
	if ( lv == K + 1 ) {
		ret = (double) ( ret + tmp );
		//fprintf(stderr,"%lf\n",tmp);
	}
	else 
		for (i=last+1;i<=N;++i)
			if (N-i>=K-lv)
				go(lv+1,i,(double)( tmp*v[i] ));
}

int main() {
int i;
	freopen(fin,"r",stdin); freopen(fout,"w",stdout);

	scanf("%d%d",&N,&K);

	for (i=1;i<=N;++i)
		scanf("%lf",&v[i]);

	go(1,0,1.0);
	
	for (i=K+1;i<=N;++i) 
		ret = (double) ( (double) ret / (double) i );
	
	printf("%.6lf\n",ret);
	

	return 0;
}