#include <bits/stdc++.h>
using namespace std;

ifstream fin("medie.in");
ofstream fout("medie.out");

int n;
int v[9005];
int fr[7005];

int main()
{
    fin>>n;
    for(int i=1; i<=n; i++)
        fin>>v[i], fr[v[i]]++;
    int s = 0;
    for(int i=1; i<n; i++)
        for(int j=i+1; j<=n; j++)
            if((v[i] + v[j]) % 2 == 0)
            {
                int val = (v[i] + v[j]) / 2;
                if(v[i] == v[j])
                    s += fr[val] - 2;
                else
                    s += fr[val];
            }
    fout<<s;
    return 0;
}