foo.cc: In function 'int main()':
foo.cc:28:1: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
   28 | cin >> a >> b;
      | ^~~
      | std::cin
In file included from /usr/include/x86_64-linux-gnu/c++/9/bits/stdc++.h:75,
                 from foo.cc:23:
/usr/include/c++/9/iostream:60:18: note: 'std::cin' declared here
   60 |   extern istream cin;  /// Linked to standard input
      |                  ^~~
foo.cc:29:1: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   29 | cout << a + b;
      | ^~~~
      | std::cout
In file included from /usr/include/x86_64-linux-gnu/c++/9/bits/stdc++.h:75,
                 from foo.cc:23:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~

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