Impreza

#pragma GCC optimize("O3")
#include <bits/stdc++.h>

bool matrix[3030][3030];
bool erased[3030];

int32_t main()
{
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(NULL);
  std::cout.tie(NULL);

  int friends_number; std::cin >> friends_number;
  int pairs_number; std::cin >> pairs_number;

  for (size_t i = 0; i < pairs_number; ++i)
  {
    int first, second; std::cin >> first >> second;
    matrix[--first][--second] = matrix[second][first] = true;
  }

  for (size_t i = 0; i < friends_number; ++i)
  {
    for (size_t j = i + 1; j < friends_number & !erased[i]; ++j)
    {
      if (!matrix[i][j] && !erased[j])
      {
        erased[i] = erased[j] = true;
      }
    }
  }

  int output_size = friends_number / 3;

  for (size_t i = 0; output_size && i < friends_number; ++i)
  {
    if (!erased[i])
    {
      --output_size;
      std::cout  << i + 1 << ' ';
    }
  }
  std::cout << '\n';
  return 0;
}