#include <bits/stdc++.h>

using namespace std;

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

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

int main()
{
    int n;
    f>>n;
    int i;
    for(i=1;i<=n;i++)
    {
        f>>v[i];
        fr[v[i]]++;
    }

    int j;
    int nr=0;
    for(i=1;i<n;i++)
    {
        for(j=i+1;j<=n;j++)
        {
            if(fr[(v[i]+v[j])/2]!=0 && (v[i]+v[j])%2==0)
                nr=nr+fr[(v[i]+v[j])/2];
        }
    }
    g<<nr;
    return 0;
}