#include<cstdio>

#define NMAX 9005

FILE *f=fopen("medie.in","r");
FILE *g=fopen("medie.out","w");

using namespace std;

int v[NMAX],frecv[NMAX],n;
int num[NMAX];

int main()
{
	fscanf(f,"%d",&n);
	
	for(int i(1); i <= n ; ++i )
	fscanf(f,"%d",&v[i]),num[v[i]]++;
	
		
	for( int i(1); i<= n-1 ; ++i )
		for(int ii (i+1); ii <= n; ++ii )
			 if( (v[i]+v[ii]) % 2 == 0 )
				frecv[(v[i]+v[ii])/2]++;
			
	int result(0);

    for( int j(1); j <= n ; ++j )
			result+=(  frecv[v[j]] -num[v[j]] +1  );
	
	fprintf(g,"%d \n",result);

	fclose(f);
	fclose(g);
	return 0;
		
}