Language: C++

#include <iostream>
#include <string>

int main(void){
	string s,a="";
	cin >> s;
	for(int i=0;i<s.length();i++){
		if(s[i]==a){
			cout << "Bad" << endl;
			return 0;
		}
			a=s[i];
	}
	cout << "Good" << endl;
	
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:5:9: error: 'string' was not declared in this scope
    5 |         string s,a="";
      |         ^~~~~~
a.cc:5:9: note: suggested alternatives:
In file included from /usr/include/c++/14/iosfwd:41,
                 from /usr/include/c++/14/ios:40,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/stringfwd.h:77:33: note:   'std::string'
   77 |   typedef basic_string<char>    string;
      |                                 ^~~~~~
In file included from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44:
/usr/include/c++/14/string:76:11: note:   'std::pmr::string'
   76 |     using string    = basic_string<char>;
      |           ^~~~~~
a.cc:6:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    6 |         cin >> s;
      |         ^~~
      |         std::cin
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:6:16: error: 's' was not declared in this scope
    6 |         cin >> s;
      |                ^
a.cc:8:26: error: 'a' was not declared in this scope
    8 |                 if(s[i]==a){
      |                          ^
a.cc:9:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    9 |                         cout << "Bad" << endl;
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:9:42: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    9 |                         cout << "Bad" << endl;
      |                                          ^~~~
      |                                          std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:12:25: error: 'a' was not declared in this scope
   12 |                         a=s[i];
      |                         ^
a.cc:14:9: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   14 |         cout << "Good" << endl;
      |         ^~~~
      |         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:14:27: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   14 |         cout << "Good" << endl;
      |                           ^~~~
      |                           std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~