foo.cc: In function 'int main()':
foo.cc:5:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    5 |         cin>>xa>>ya>>xb>>yb;
      |         ^~~
      |         std::cin
In file included from /nix/store/wy4ywjsch9q2hj5lphqjdg9p2kf7w0ls-gcc-11.3.0/include/c++/11.3.0/x86_64-unknown-linux-gnu/bits/stdc++.h:75,
                 from foo.cc:1:
/nix/store/wy4ywjsch9q2hj5lphqjdg9p2kf7w0ls-gcc-11.3.0/include/c++/11.3.0/iostream:60:18: note: 'std::cin' declared here
   60 |   extern istream cin;           /// Linked to standard input
      |                  ^~~

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