Language: C++

#include<iostream>
#include<string>
using namespace std;
int main() {
	string A, B, C;
	cin >> A >> B >> C;
	if (A.at(A) == B.at(0) && B.at(B) == C.at(0)) {
		cout << "YES" << endl;
	}
	else cout << "NO" << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:7:17: error: no matching function for call to 'std::__cxx11::basic_string<char>::at(std::string&)'
    7 |         if (A.at(A) == B.at(0) && B.at(B) == C.at(0)) {
      |             ~~~~^~~
In file included 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/c++/14/bits/basic_string.h:1287:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::const_reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::at(size_type) const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; const_reference = const char&; size_type = long unsigned int]'
 1287 |       at(size_type __n) const
      |       ^~
/usr/include/c++/14/bits/basic_string.h:1287:20: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1287 |       at(size_type __n) const
      |          ~~~~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:1309:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::at(size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]'
 1309 |       at(size_type __n)
      |       ^~
/usr/include/c++/14/bits/basic_string.h:1309:20: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1309 |       at(size_type __n)
      |          ~~~~~~~~~~^~~
a.cc:7:39: error: no matching function for call to 'std::__cxx11::basic_string<char>::at(std::string&)'
    7 |         if (A.at(A) == B.at(0) && B.at(B) == C.at(0)) {
      |                                   ~~~~^~~
/usr/include/c++/14/bits/basic_string.h:1287:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::const_reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::at(size_type) const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; const_reference = const char&; size_type = long unsigned int]'
 1287 |       at(size_type __n) const
      |       ^~
/usr/include/c++/14/bits/basic_string.h:1287:20: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1287 |       at(size_type __n) const
      |          ~~~~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:1309:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::at(size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]'
 1309 |       at(size_type __n)
      |       ^~
/usr/include/c++/14/bits/basic_string.h:1309:20: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1309 |       at(size_type __n)
      |          ~~~~~~~~~~^~~