#include <fstream>

using namespace std;
int f[7005], v[9005], m[9005];
int main()
{
    ifstream cin("medie.in");
    ofstream cout("medie.out");
    int n, a, h = 1, st, dr, mid, cnt = 0, cz;
    double p;
    cin >> n;

    for(int i = 1; i <= n; i++)
    {
        cin >> v[i];
        f[v[i]]++;
    }

    for(int i = 1; i < 7005; i++)
    {
        if(f[i] > 0)
        {
            m[h] = i;
            h++;
        }
    }

    for(int i = 1; i <= n; i++)
    {
        for(int j = i + 1; j <= n; j++)
        {
            p = v[i] + v[j];
            p /= 2;

            if((int)(p * 10) % 10 == 0 && f[(int)p] > 0)
            {
                cz = 0;

                if(v[i] == p)
                    cz++;

                if(v[j] == p)
                    cz++;

                cnt += f[(int)p] - cz;
            }
        }
    }

    cout << cnt;
    return 0;
}