#include<cstdio>
double rez,a[26],st[26];
int n,k,nr;
void comb(int i,double rez1)
{
	if(i==k+1) {rez+=rez1;nr++;return;}
	int max=n-(k-i);
	for(int j=st[i-1]+1;j<=max;j++){
		st[i]=j;
		comb(i+1,rez1*a[j]);}
}
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",&a[i]);
	comb(1,1.0);
	printf("%.6lf",rez/nr);
	fclose(stdout);
	return 0;
}