#include <fstream>

using namespace std;

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

int i,j,n,m,af,c,a[9005],v[9005];

int main()
{
    fin >> n;
    for (i=1;i<=n;i++)
    {
        fin >> a[i];
        v[a[i]]++;
    }
    for (i=1;i<=n;i++)
    {
        for (j=i+1;j<=n;j++) if ((a[i]+a[j])%2==0)
        {
            m=(a[i]+a[j])/2;
            c=v[m];
            if (a[i]==m) c--;
            if (a[j]==m) c--;
            if (c>0) af+=c;
        }
    }
    fout << af;

    return 0;
}