/in/foo.cc: In function 'void PLUS(int, int)':
/in/foo.cc:6:23: error: return-statement with a value, in function returning 'void' [-fpermissive]
  return firstNumber + SecNumber;
                       ^~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:11:36: error: 'endl' was not declared in this scope
     std::cout << "hello, world" << endl;
                                    ^~~~
/in/foo.cc:11:36: note: suggested alternative:
In file included from /usr/include/c++/6/iostream:39:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~