Language: C++

#include <iostream>
#include <fstream>
#include <stdio.h>
#include <math.h>
#include <time.h>
#include <string>
#include <tuple>
#include <vector>
#include <map>
#include <unordered_map>
#include <list>
#include <set>
#include <stack>
#include <queue>
#include <cstdlib>
#include <algorithm>
#include <random>
#include <cassert>
using namespace std;
#define LL long long
#define MP(a, b) make_pair(a, b)
#define MMP(a, b, c) make_pair(make_pair(a, b), c)
#define MAX 1000000000
#undef INT_MIN
#undef INT_MAX
#define INT_MIN -2147483647
#define INT_MAX 2147483647
#define LL_MIN (LL)-9223372036854775807
#define LL_MAX (LL)9223372036854775807
#define PI 3.14159265359

int main(){
    iostream::sync_with_stdio(false);
    
    string S;
    cin >> S;
    S = S.substr(0, S.size()-1);
    if(S.size() % 2 == 1) S = S.substr(0, S.size()-1);
    
    while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
        S = S.substr(0,S.size()-2);
    }
    cout << S.size() << endl;

    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:40:33: error: no match for 'operator!=' (operand types are 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} and 'std::__cxx11::basic_string<char>')
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                      ~~~~~~~~~~ ^~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                              |             |
      |                              |             std::__cxx11::basic_string<char>
      |                              std::__cxx11::basic_string<char>::size_type {aka long unsigned int}
In file included from /usr/include/c++/14/iosfwd:42,
                 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/postypes.h:197:5: note: candidate: 'template<class _StateT> bool std::operator!=(const fpos<_StateT>&, const fpos<_StateT>&)'
  197 |     operator!=(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
      |     ^~~~~~~~
/usr/include/c++/14/bits/postypes.h:197:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::fpos<_StateT>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
In file included from /usr/include/c++/14/string:43,
                 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/bits/allocator.h:243:5: note: candidate: 'template<class _T1, class _T2> bool std::operator!=(const allocator<_CharT>&, const allocator<_T2>&)'
  243 |     operator!=(const allocator<_T1>&, const allocator<_T2>&)
      |     ^~~~~~~~
/usr/include/c++/14/bits/allocator.h:243:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::allocator<_CharT>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
In file included from /usr/include/c++/14/string:48:
/usr/include/c++/14/bits/stl_iterator.h:455:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  455 |     operator!=(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:455:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/bits/stl_iterator.h:500:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  500 |     operator!=(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:500:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::reverse_iterator<_Iterator>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1686 |     operator!=(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1686:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator!=(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1753 |     operator!=(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1753:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::move_iterator<_IteratorL>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_pair.h:1052:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator!=(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1052 |     operator!=(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1052:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::pair<_T1, _T2>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:651:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  651 |     operator!=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:651:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'long unsigned int'
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/string_view:658:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  658 |     operator!=(basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:658:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'long unsigned int'
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/string_view:666:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator!=(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  666 |     operator!=(__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:666:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/bits/basic_string.h:3833:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3833 |     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3833:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/bits/basic_string.h:3847:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3847 |     operator!=(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3847:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const _CharT*' and 'long unsigned int'
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
/usr/include/c++/14/bits/basic_string.h:3860:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator!=(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3860 |     operator!=(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3860:5: note:   template argument deduction/substitution failed:
a.cc:40:66: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'}
   40 |     while(S.substr(0,S.size()/2 != S.substr(S.size()/2,S.size()/2){
      |                                                                  ^
In file included from /usr/