#include <iostream>
#include <fstream>
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");
int v[7001];
int main()
{int i,j,n,N=0;
long long nr=0;
f>>n;
for(i=1;i<=n;i++)
{
    f>>j;
    if(j>N) N=j;
    v[j]++;
}
for(i=1;i<=N;i++)
    if(v[i]){
    if(v[i]>2){
    if(v[i]%2)
    nr+=((v[i]-2)*((v[i]-1)/2)*v[i]);
    else nr+=((v[i]-2)*(v[i]/2)*(v[i]-1));}
    for(j=i+1;j<=N;j++)
    if(!((i+j)%2) and v[j] and v[(i+j)/2])
    nr+=min(min(v[i],v[j]),v[(i+j)/2]);
    }
g<<nr;

    return 0;
}