#include <stdio.h>
#define NM 9001

int n, m;
int i, j, k;
int v[NM], fr[NM];
int nrsol, p1, p2;

int main()
{
    freopen("medie.in", "r", stdin);
    freopen("medie.out", "w", stdout);
    scanf("%d", &n);
    for ( i = 1; i <= n; i++ ) scanf("%d", &v[i]), fr[v[i]]++;
    
    for ( i = 1; i <= n; i++ )
		for ( j = i+1; j <= n; j++ )
            if ( (v[i]+v[j])%2 == 0 )
            {
                k = (v[i]+v[j])/2;
                nrsol += fr[k];
                if ( v[i] == v[j] ) nrsol -= 2;    
            }    
    printf("%d\n", nrsol);
    return 0;
}