#include <bits/stdc++.h>
#define Roc ios_base::sync_with_stdio(false), cin.tie(NULL)
#define ll long long
#define NMAX 9000 + 10
#define pi pair<ll,ll>
using namespace std;

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

int n, v[NMAX], fr[NMAX], m[NMAX],ans;

int main()
{
    Roc;
    f >> n;
    for(int i = 1; i <= n; ++i)
    {
        f >> v[i];
        fr[ v[i] ]++;
    }
    for(int i = 1; i <= n; ++i)
        for(int j = i + 1; j <= n; ++j)
            if( (v[i] + v[j]) % 2 == 0 )
                m[ (v[i] + v[j]) / 2 ]++;
    for(int i = 1; i <= n; ++i)
            ans += m[ v[i] ] - fr[ v[i] ] + 1;
    g << ans;
    return 0;
}