#include <fstream>

using namespace std;

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

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

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

    return 0;
}