dobri

#include <iostream>
#include <unordered_set>
using namespace std;
int N;

unordered_set<int> s2;

int x[5050];

int main(){
  cin >> N;
  int ans=0;
  for (int i=0; i<N; ++i){
    cin >> x[i];
    bool good=0;
    for (int j=0; j<i; ++j){
      if (s2.count(x[i]-x[j])){
	good=1;
	break;
      }
    }
    for (int j=0; j<=i; ++j){
      s2.insert(x[i]+x[j]);
    }
    ans+=good;
  }
  cout << ans << "\n";
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED