#include <bits/stdc++.h>
#define hg 1<<10
#define verf ++poz == hg ? fin.read (ch, hg), poz = 0 : 0
#define fm 7001
#define nm 9001

using namespace std;

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

int fv[fm],v[nm],n,poz;
long long rez;
char ch[hg];

inline void read( int &x )
{
    if (ch[0] == '\0') fin.read (ch, hg) ;
    else for (; ch[poz] < '0' || ch[poz] > '9' ; verf) ;
    for (x = 0 ; ch[poz] >= '0' && ch[poz] <= '9' ; x = x * 10 + ch[poz] - '0', verf) ;
}

int main()
{
    int i,j;
    read(n);
    for(i=1;i<=n;i++)
    {
        read(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;
}