#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
vector<int> v;
long long n,i,x,nr,j,s[9001];
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
    {
       fin>>x;
       v.push_back(x);
       s[x]++;
    }
    for(i=0;i<n-1;i++)
        for(j=i+1;j<n;j++)
          {
              if((v[i]+v[j])%2==0)
              {
                  nr+=s[(v[i]+v[j])/2];
              if(v[i]==(v[i]+v[j])/2) nr-=2;
              }

          }
    fout<<nr;
    return 0;
}