#include<cstdio>
#include<algorithm>
#define maxn 9001

using namespace std;

int v[maxn],hash[maxn],i,j,n,rez;

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]),
		hash[v[i]]++;
	
	sort ( v + 1 , v + n + 1 );
		
	for( i = 1 ; i <= n ; ++i )
		for( j = i + 1 ; j <= n ; ++j )
			if ( ( v[i] + v[j] & 1 ) == 0 )
			{ 
			rez += hash[( v[i] + v[j]) /2];
			if ( v[i] == ( v[i] + v[j] ) / 2 ) rez -=2;
			}
		
printf("%d\n",rez);
return 0;
}