#include <fstream>
using namespace std;
ifstream fin ("medie.in");
ofstream fout ("medie.out");
int n,maxim,v[9001],fr[7001],sol[7001];
long long nrc;
int main()
{
    fin>>n;
    for (int i=1; i<=n; i++)
    {
        fin>>v[i];
        fr[v[i]]++;
        maxim=max (maxim,v[i]);
    }
    for (int i=1; i<=maxim; i++)
    {
        for (int j=i+1; j<=maxim; j++)
        {
            if ((i+j)%2==0)
                sol[(i+j)/2]+=fr[i]*fr[j];
        }
    }
    for (int i=1; i<=n; i++)
        nrc+=sol[v[i]]+1ll*(fr[v[i]]-1)*(fr[v[i]]-2)/2;
    fout<<nrc;
    return 0;
}