#include <bits/stdc++.h>
#define NMAX 9005
#define MAX_NUM 7005

using namespace std;

/*******************************/
// INPUT / OUTPUT

ifstream f("medie.in");
ofstream g("medie.out");
/*******************************/
/// GLOBAL DECLARATIONS

int N;
int ans;
int v[NMAX], freq[2 * MAX_NUM];
/*******************************/
/// FUNCTIONS

void ReadInput();
void Solution();
void Output();
/*******************************/
///-------------------------------------
inline void ReadInput()
{
    f >> N;

    for (int i = 1 ; i <= N ; ++ i)
    {
        f >> v[i];
        freq[2 * v[i]] ++;
    } 
}
///-------------------------------------
inline void Solution()
{
    int x;
    for (int i = 1 ; i <= N ; ++ i)
    {
        for (int j = i + 1 ; j <= N ; ++ j)
        {
            x = freq[v[i] + v[j]];
            if (v[i] == v[j]) x -= 2;
            ans += max(0, x);
        }
    }
}
///-------------------------------------
inline void Output()
{
    g << ans;
}
///-------------------------------------
int main()
{
    ReadInput();
    Solution();
    Output();
    return 0;
}