# include <stdio.h>
using namespace std;
int a[9003], n, sol, m, b[7003];
void citire ()
{
	int i;
	freopen ("medie.in", "r", stdin);
	scanf ("%d", &n);
	for (i=1;i<=n;i++)
		scanf ("%d", &a[i]), b[a[i]]=1;
}
void medie ()
{
	int i, j, m;
	for (i=1;i<n;i++)
		for (j=i+1;j<=n;j++)
	  	    if ((a[i]+a[j])%2==0)
			{
			    m=(a[i]+a[j])/2;
			    if (b[m]) sol++;
			}
}		 
int main ()
{
	citire ();
	freopen ("medie.out", "w", stdout);
	for (int i=1;i<=n;i++)
		printf ("%d %d\n", a[i], b[a[i]]);
	medie ();
	printf ("%d", sol);
	return 0;
}