#include<iostream>
#include<fstream>
#include<map>
using namespace std;

ifstream f("medie.in");
ofstream g("medie.out");

int n,v[9005],frev[14005];

void citire()
{
    f>>n;
    for(int i=1;i<=n;i++)
    {
        f>>v[i];
        frev[v[i]*2]++;
    }
}

void solve()
{
    int cnt = 0;
    for(int i=1;i<n;i++)
        for(int j=i+1;j<=n;j++)
            cnt+=max(0,frev[v[i] + v[j]] - 2*(v[i] == v[j]));
    g<<cnt;
}

int main()
{
    citire();
    solve();
}