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

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

void go(int lv,int last,long double tmp) {
int i;
	if ( lv == K + 1 ) {
		for (i=K+1;i<=N;++i)
			tmp=(long double)tmp/i;
		ret = (long double) ret + tmp;
	}
	else 
		for (i=last+1;i<=N;++i)
			go(lv+1,i,(long 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("%llf",&v[i]);
	
	go(1,0,1.0);

	printf("%llf\n",ret);

	return 0;
}