Language: C++

#include <iostream>
using namespace std;
int main(){
  int N;
  cin >> N;
  int keta = 0;
  int count = 0;
  int s, t;
  string M;
  for(int i=1;i<10;++i) {
    for(int j=1;j<10;++j) {
      for(int k=1;k<=N;++k) {
        M = to_string(k);
        s = M[M.begin()] - '0';
        t = M[M.end()] - '0';
        if(i==s&&j==t) ++count;
        if(j==s&&i==t) ++count;
      }
    }
  }
  cout << count;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:14:14: error: no match for 'operator[]' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'std::__cxx11::basic_string<char>::iterator')
   14 |         s = M[M.begin()] - '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:1247:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::const_reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; const_reference = const char&; size_type = long unsigned int]'
 1247 |       operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1247:29: note:   no known conversion for argument 1 from 'std::__cxx11::basic_string<char>::iterator' to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1247 |       operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
      |                   ~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:1265:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]'
 1265 |       operator[](size_type __pos)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1265:28: note:   no known conversion for argument 1 from 'std::__cxx11::basic_string<char>::iterator' to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1265 |       operator[](size_type __pos)
      |                  ~~~~~~~~~~^~~~~
a.cc:15:14: error: no match for 'operator[]' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'std::__cxx11::basic_string<char>::iterator')
   15 |         t = M[M.end()] - '0';
      |              ^
/usr/include/c++/14/bits/basic_string.h:1247:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::const_reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; const_reference = const char&; size_type = long unsigned int]'
 1247 |       operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1247:29: note:   no known conversion for argument 1 from 'std::__cxx11::basic_string<char>::iterator' to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1247 |       operator[] (size_type __pos) const _GLIBCXX_NOEXCEPT
      |                   ~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:1265:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator[](size_type) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&; size_type = long unsigned int]'
 1265 |       operator[](size_type __pos)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1265:28: note:   no known conversion for argument 1 from 'std::__cxx11::basic_string<char>::iterator' to 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
 1265 |       operator[](size_type __pos)
      |                  ~~~~~~~~~~^~~~~