#include <iostream>
#include <fstream>
#define NMax 9002
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");
int n,v[NMax],h[NMax];
int main ()
{
    int i,j,ct=0;
    f>>n;
    for (i=1;i<=n;i++)
    {
        f>>v[i];
        h[v[i]]=1;
    }
    for (i=1;i<=n-1;i++)
        for (j=i+1;j<=n;j++)
            if ((v[i]+v[j])/2<=7000)
                if ((float)(v[i]+v[j])/2==(v[i]+v[j])/2)
                    if (h[(v[i]+v[j])/2])
                        ct++;
    g<<ct;
}