#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;
long long sum(int s)
{
    hasu.clear();
    long long ans = 0;
    for(int i = 1; i <= n; ++i)
    {
        ans += hasu[s - v[i]];
        hasu[v[i]]++;
    }
    return ans;
}
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];
    }
    for(int i = 1; i <= n; ++i)
    {
        sol += sum(2 * v[i]);
    }
    fout << sol;
    return 0;
}