#include <iostream>
#include <fstream>
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");
 int n,v[9005],s[15005],ap[15005];
 long long sol=0;
int main()
{    int i,j;
    f>>n;

   for(i=1;i<=n;i++)
    {f>>v[i];
     ap[v[i]]++;
    }

   for(i=1;i<=n;i++)
    for(j=i+1;j<=n;j++)
     s[v[i]+v[j]]++;

   for(i=1;i<=n;i++)
    sol+=s[v[i]*2]-(ap[v[i]]-1)*(ap[v[i]]>1);

  g<<sol;

    return 0;
}