#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;
ifstream in ("medie.in");
ofstream out ("medie.out");
int const nmax = 9000;
int frec[5 + nmax];
int v[5 + nmax];

int main()
{
  int n;
  in>>n;
  for(int i = 1 ; i <= n ;i++){
    in>>v[i];
    frec[v[i]]++;
  }
  int result = 0;
  for(int i = 1 ; i <= n ;i++){
    for(int j = i + 1 ; j <= n ;j++){
      int mid = (v[i] + v[j]) / 2;
      if(v[i] + v[j] == mid * 2){
        result += frec[mid];
        if(mid == v[i])
          result--;
        if(mid == v[j])
          result--;
      }
    }
  }
  out<<result;
  return 0;
}