foo.cc: In function 'int main()':
foo.cc:19:2: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
   19 |  cin >> n;
      |  ^~~
      |  std::cin
In file included from foo.cc:4:
/usr/include/c++/9/iostream:60:18: note: 'std::cin' declared here
   60 |   extern istream cin;  /// Linked to standard input
      |                  ^~~
foo.cc:28:2: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   28 |  cout << k;
      |  ^~~~
      |  std::cout
In file included from foo.cc:4:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~

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