#include <fstream>

using namespace std;

int frec[7005];

int main()
{
    ifstream cin("medie.in");
    ofstream cout("medie.out");
    
    int n=0,i,j,rasp=0,v[9005];
    cin>>n;
    
    for(i=0;i<n;i++)
    {
       cin>>v[i];
       frec[v[i]<<1]++;
    }
    for(i=0;i<n;i++)
       for(j=i+1;j<n;j++)
       {
           rasp+=frec[v[i]+v[j]];
           if(v[i]==v[j])
              rasp-=2;     
       }
    cout<<rasp<<'\n';
    cin.close();
    cout.close();
    //system("PAUSE");
    return 0;
}