#include <stdio.h>
#define NMax 10000

int n;
double a[NMax];
double m[NMax][NMax];
FILE *f, *g;

void citire();
void rez();

int main()
{
	citire();
   rez();
	return 0;
}
void rez()
{
   int i, j, k, pmax=0;

   for (i=0; i<n-1; i++)
       for (j=i+1; j<n; j++)
           if ( i != j )
              m[i][j] = (a[i]+a[j])/2.0;

   for (i=0; i<n; i++)
   for (j=0; j<n; j++)
   for (k=0; k<n; k++)
   if ( i!=j && i!=k && j!=k )
      if ( m[j][k] == a[i] )
         pmax++;
   fprintf( g, "%d", pmax );
}
void citire()
{
	int i;
	f = fopen( "medie.in", "rt" );
	g = fopen( "medie.out", "wt");
	fscanf( f, "%d", &n );

	for (i=0; i<n; i++)
       fscanf( f, "%lf", &a[i] );
}