Language: C++

#include <iostream>
#include <vector>

int main() {
  int n = 0;
  cin >> n;
  vector<int> in(n);
  vector<bool> check(n, false);
  for (int i=0; i<n; i++) {
    cin >> in.at(i);
    in.at(i)--;
  }
  int count = 0;
  int now = 0;
  check.at(0) = true;
  while (true) {
  	now = a.at(now);
    count++;
    if (now == 1) {
      cout << count << endl;
      return 0;
    }
    else if (check.at(now)) {
      cout << -1 << endl;
      return 0;
    }
    check.at(now) = true;
  }
  return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:6:3: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    6 |   cin >> n;
      |   ^~~
      |   std::cin
In file included from a.cc:1:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:7:3: error: 'vector' was not declared in this scope
    7 |   vector<int> in(n);
      |   ^~~~~~
a.cc:7:3: note: suggested alternatives:
In file included from /usr/include/c++/14/vector:66,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_vector.h:428:11: note:   'std::vector'
  428 |     class vector : protected _Vector_base<_Tp, _Alloc>
      |           ^~~~~~
/usr/include/c++/14/vector:93:13: note:   'std::pmr::vector'
   93 |       using vector = std::vector<_Tp, polymorphic_allocator<_Tp>>;
      |             ^~~~~~
a.cc:7:10: error: expected primary-expression before 'int'
    7 |   vector<int> in(n);
      |          ^~~
a.cc:8:10: error: expected primary-expression before 'bool'
    8 |   vector<bool> check(n, false);
      |          ^~~~
a.cc:10:12: error: 'in' was not declared in this scope; did you mean 'i'?
   10 |     cin >> in.at(i);
      |            ^~
      |            i
a.cc:15:3: error: 'check' was not declared in this scope
   15 |   check.at(0) = true;
      |   ^~~~~
a.cc:17:15: error: 'a' was not declared in this scope
   17 |         now = a.at(now);
      |               ^
a.cc:20:7: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   20 |       cout << count << endl;
      |       ^~~~
      |       std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:20:24: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   20 |       cout << count << endl;
      |                        ^~~~
      |                        std::endl
In file included from /usr/include/c++/14/iostream:41:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:24:7: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   24 |       cout << -1 << endl;
      |       ^~~~
      |       std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:24:21: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   24 |       cout << -1 << endl;
      |                     ^~~~
      |                     std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~