#include<stdio.h>
FILE *in=fopen("dezastru.in","r"),*out=fopen("dezastru.out","w");
int n,k,nr=1,i,st[30],ok=0;
double s=0,v[30],mat[26][26];
void bktr(int p)
{
	for(float pval=st[p-1]+1;pval<=n;pval++)
	{
		st[p]=pval;
		if(p==k)
		{
			mat[nr][0]=1;
			for(int x=1;x<=k;x++)
			{
				mat[nr][0]*=v[st[x]];
			}
			nr++;ok=1;
		}
		else bktr(p+1);
	}
}
int main()
{
	fscanf(in,"%d %d",&n,&k);
	for(i=1;i<=n;i++)
		fscanf(in,"%lf ",&v[i]);
	for(i=1;i<=n+1;i++)
		st[i]=0;
	bktr(1);
	for(i=1;i<=nr;i++)
		s+=mat[i][0];
	if(!ok)
		fprintf(out,"%.6lf",s/nr);
	else {nr--;fprintf(out,"%.6lf",s/nr);}
	fcloseall();
	return 0;
}