#include <cstdio>
#include <vector>
#include <queue>
#include <stack>
#include <list>
#include <set>
#include <algorithm>
#include <utility>
#include <string>
#include <functional>
#include <sstream>
#include <fstream>
#include <iostream>
using namespace std;
#define FOR(i,a,b) for (i=a;i<=b;i++)
#define fori(it,v) for (it=(v).begin();it!=(v).end();it++)
#define pb push_back
#define mp make_pair
#define fs first
#define ss second
#define all(c) c.begin(),c.end()
#define pf push_front
#define popb pop_back
#define popf pop_front
FILE *in,*out;
int k,n,pos;
double sum,a[40];
void ceva(int x,int y,double z)
{
	int i;
	if (x==k+1)
	{
		pos++;
		sum+=z;
		return ;
	}
	int r=n-k+x;
	FOR(i,y+1,r)
		ceva(x+1,i,z*a[i]);
}
int main()
{
	int i;
	in=fopen("dezastru.in","r");
	out=fopen("dezastru.out","w");
	fscanf(in,"%d%d",&n,&k);
	FOR(i,1,n)
	{
		fscanf(in,"%lf",&a[i]);
	}
	ceva(1,0,1);
	fprintf(out,"%lf\n",sum/pos);
	fclose(in);
	fclose(out);
	return 0;
}