foo.cc: In function 'int main()':
foo.cc:6:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    6 |         cin>>a;
      |         ^~~
      |         std::cin
In file included from stdc++.h:75:
/nix/store/42izybv2fz3ps5pcg3dvcmn85pc9406y-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
      |                  ^~~
foo.cc:13:9: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   13 |         cout<<cnt;
      |         ^~~~
      |         std::cout
In file included from stdc++.h:75:
/nix/store/42izybv2fz3ps5pcg3dvcmn85pc9406y-gcc-11.3.0/include/c++/11.3.0/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;          /// Linked to standard output
      |                  ^~~~

[Hydro](https://hydro.ac)提供评测服务