#include <fstream>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
int s,n,m,s1,v[9004],a[9004],v2[9004],ma,t;
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>a[i];
        v[a[i]]++;
        m=max(m,a[i]);
        for(int j=1;j<i;j++)
        {
            if((a[i]+a[j])%2==1)
                continue;
            v2[(a[i]+a[j])/2]++;
        }
    }
    for(int i=1;i<=n;i++)
        s+=v2[a[i]]-v[a[i]]+1;
    fout<<s;
    return 0;
}