#include <fstream>

using namespace std;

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

int n,i,j,v[9005],f[7005];

int main()
{
    fin >> n;
    for (i=1; i<=n; i++)
    {
        fin >> v[i];
        f[v[i]]++;
    }
    long long sol = 0;
    for (i=1; i<n; i++)
        for (j=i+1; j<=n; j++)
            if ((v[i]+v[j])%2 == 0 && f[(v[i]+v[j])/2])
            {
                int val = (v[i]+v[j])/2; int ok = 0;
                if (v[i] == val)
                    ok++;
                if (v[j] == val)
                    ok++;
                sol += f[val]-ok;
            }
    fout << sol;
    return 0;
}