#include <fstream>
#define VAL 9005

using namespace std;

ifstream fin("medie.in");
ofstream fout("medie.out");

int N, M, i;
int mx, v[VAL];
int a[VAL], j;

int main()
{
    fin >> N;
    for (i=1; i<=N; i++)
    {
        fin >> v[i];
        a[v[i]]++;
    }
    for (i=1; i<N; i++)
    {
        for (j=i+1; j<=N; j++)
        {
            M=v[i]+v[j];
            M/=2;
            if (M*2==v[i]+v[j])
            {
                mx+=a[M];
                if (v[i]==M)
                  mx-=2;
            }
        }
    }
    fout << mx << '\n';
    fin.close();
    fout.close();
    return 0;
}