Language: C++

#include <iostream>
#include <stdio.h>
using namespace std;

int main() {
	string a,b;
	cin >> a >> b;
	if (a.size() < b.size()) {
		cout << "LESS" << endl;
	} else if (a.size() > b.size()) {
		cout << "GREATER" << endl;
	} else {
		for (int i = 0;i < a.size();i++) {
			if (atoi(a[i]) < atoi(b[i])) {
				cout << "LESS" << endl;
				break;
			} else if (atoi(a[i]) > atoi(b[i])) {
				cout << "GREATER" << endl;
				break;
			} else {
				cout << "EQUAL" << endl;
				break;
			}
		}
	}
}

Compiler output:
a.cc: In function 'int main()':
a.cc:14:33: error: invalid conversion from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const char*' [-fpermissive]
   14 |                         if (atoi(a[i]) < atoi(b[i])) {
      |                             ~~~~^~~~~~
      |                                 |
      |                                 __gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type {aka char}
In file included from /usr/include/c++/14/cstdlib:79,
                 from /usr/include/c++/14/ext/string_conversions.h:43,
                 from /usr/include/c++/14/bits/basic_string.h:4154,
                 from /usr/include/c++/14/string:54,
                 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,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/stdlib.h:105:30: note:   initializing argument 1 of 'int atoi(const char*)'
  105 | extern int atoi (const char *__nptr)
      |                  ~~~~~~~~~~~~^~~~~~
a.cc:14:46: error: invalid conversion from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const char*' [-fpermissive]
   14 |                         if (atoi(a[i]) < atoi(b[i])) {
      |                                          ~~~~^~~~~~
      |                                              |
      |                                              __gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type {aka char}
/usr/include/stdlib.h:105:30: note:   initializing argument 1 of 'int atoi(const char*)'
  105 | extern int atoi (const char *__nptr)
      |                  ~~~~~~~~~~~~^~~~~~
a.cc:17:40: error: invalid conversion from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const char*' [-fpermissive]
   17 |                         } else if (atoi(a[i]) > atoi(b[i])) {
      |                                    ~~~~^~~~~~
      |                                        |
      |                                        __gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type {aka char}
/usr/include/stdlib.h:105:30: note:   initializing argument 1 of 'int atoi(const char*)'
  105 | extern int atoi (const char *__nptr)
      |                  ~~~~~~~~~~~~^~~~~~
a.cc:17:53: error: invalid conversion from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const char*' [-fpermissive]
   17 |                         } else if (atoi(a[i]) > atoi(b[i])) {
      |                                                 ~~~~^~~~~~
      |                                                     |
      |                                                     __gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type {aka char}
/usr/include/stdlib.h:105:30: note:   initializing argument 1 of 'int atoi(const char*)'
  105 | extern int atoi (const char *__nptr)
      |                  ~~~~~~~~~~~~^~~~~~