/in/foo.cc:2:5: warning: 'int main(int)' takes only zero or two arguments [-Wmain]
 int main(int a){cin>>a;cout<<a*a-2*a;}
     ^~~~
/in/foo.cc: In function 'int main(int)':
/in/foo.cc:2:17: error: 'cin' was not declared in this scope
 int main(int a){cin>>a;cout<<a*a-2*a;}
                 ^~~
/in/foo.cc:2:17: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:75:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
/in/foo.cc:2:24: error: 'cout' was not declared in this scope
 int main(int a){cin>>a;cout<<a*a-2*a;}
                        ^~~~
/in/foo.cc:2:24: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:75:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~