#include <stdio.h>
#define NMax 18000

int n, p;
int a[NMax];
int med[NMax][NMax];
FILE *f, *g;

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

	for (i=0; i<n; i++)
		for (j=0; j<n; j++)
			med[i][j] = NMax;

	for (i=0; i<n-1; i++)
		for (j=i+1; j<n; j++)
			med[i][j] = (a[i]+a[j])/2;

	for (i=0; i<n; i++)
	for (j=0; j<n; j++)
	for (k=0; k<n; k++)
		if ( i != j && j != k && i != k )
		{
			if ( med[j][k] == a[i] )
				p++;
		}

	fprintf( g, "%d\n", p );
}
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, "%d" , &a[i] );
}