#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("fast-math")
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
int n, v[9005];
long long sol;
map<int, int>hasu;
int fr[7005];
ifstream fin("medie.in");
ofstream fout("medie.out");
int32_t main(int argc, char * argv[])
{
    fin >> n;
    for(int i = 1; i <= n; ++i)
    {
        fin >> v[i];
        fr[v[i]]++;
    }
    for(int i = 1; i < n; ++i)
    {
        for(int j = i + 1; j <= n; ++j)
        {
             hasu[v[i] + v[j]]++;
        }
    }
    for(int i = 1; i <= n; ++i)
    {
        if(hasu.find(2 * v[i]) != hasu.end())
        {
            sol = sol + hasu[2 * v[i]] - fr[v[i]] + 1;
        }
    }
    fout << sol;
    return 0;
}