#include <bits/stdc++.h>
#define fm 7001
#define nm 9001

using namespace std;

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

int fv[fm],v[nm],n;
long long rez;

int main()
{
    int i,j;
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>v[i];
        fv[v[i]]++;
    }
    for(i=1;i<=n;i++)
    {
        for(j=i+1;j<=n;j++)
        {
            if(!((v[i]+v[j])%2))
            {
                rez+=fv[(v[i]+v[j])/2];
                if(v[i]==v[j])
                    rez-=2;
            }
        }
    }
    fout<<rez;
    fout.close();
    return 0;
}