Language: C++

#include <bits/stdc++.h>
using namespace std;

#define inf 1072114514
#define llinf 4154118101919364364
#define mod 1000000007
#define pi 3.1415926535897932384


int round(int a,int b){if((a%b)*2 >= b){return (a/b)+1;}return a/b;} 
int gcd(int a,int b){int c;while(b!=0){c=a%b;a=b;b=c;}return a;}    //最大公約数
int lcm(int a,int b){int c=gcd(a,b);a/=c;return a*b;}     //最小公倍数
int nCr(int a,int b){int i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;} //コンビネーション
int nHr(int a,int b){return nCr(a+b-1,b);}       // 重複組み合わせ
int fact(int a){int i,r=1;for(i=1;i<=a;i++){r*=i;}return r;}  //階乗
int pow(int a,int b){int i,r=1;for(i=1;i<=b;i++){r*=a;}return r;} // a~bまでの階乗
int dsum(int x){int r=0;while(x){r+=(x%10);x/=10;}return r;}   //数字の各位の和
int dsumb(int x,int b){int r=0;while(x){r+=(x%b);x/=b;}return r;}  // b進数の各位の和？
int sankaku(int x){return ((1+x)*x)/2;} //三角数 xまでの和

//以下long long ver
long long llmax(long long a,long long b){if(a>b){return a;}return b;}
long long llmin(long long a,long long b){if(a<b){return a;}return b;}
long long llzt(long long a,long long b){return llmax(a,b)-llmin(a,b);}
long long llround(long long a,long long b){if((a%b)*2 >= b){return (a/b)+1;}return a/b;}
long long llceil(long long a,long long b){if(a%b==0){return a/b;}return (a/b)+1;}
long long llgcd(long long a,long long b){long long c;while(b!=0){c=a%b;a=b;b=c;}return a;}
long long lllcm(long long a,long long b){long long c=llgcd(a,b);a/=c;return a*b;}
long long llnCr(long long a,long long b){long long i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;}
long long llnHr(long long a,long long b){return llnCr(a+b-1,b);}
long long llfact(long long a){long long i,r=1;for(i=1;i<=a;i++){r*=i;}return r;}
long long llpow(long long a,long long b){long long i,r=1;for(i=1;i<=b;i++){r*=a;}return r;}
long long lldsum(long long x){long long r=0;while(x){r+=(x%10);x/=10;}return r;}
long long lldsumb(long long x,long long b){long long r=0;while(x){r+=(x%b);x/=b;}return r;}
long long llsankaku(long long x){return ((1+x)*x)/2;}

//double
double dbmax(double a,double b){if(a>b){return a;}return b;}
double dbmin(double a,double b){if(a<b){return a;}return b;}
double dbzt(double a,double b){return dbmax(a,b)-dbmin(a,b);}


typedef pair<int, int> ii;
typedef long long ll;
typedef vector<ll> vll;
typedef pair<ll,ll> pll;
#define forr(i,a,b) ; for(int i=(a); i<(b); i++)
#define clean(arr,val) memset(arr,val,sizeof(arr))
#define forn(i,n) forr(i,0,n)
#define PB push_back
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<pll> vpll;
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
bool is_prime (int x){
  if (x<=1) return false ;
  for (int i=2 ; i*i<=x ; i++){
    if (x%i==0) return false ;
  }
   return true;
}
string alpha = "abcdefghijklmnopqrstuvwxyz" ;
string ALPHA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ" ;
    
bool chmin (int a,int b){
    if ( a > b ){
    a = b;
    return true ;
    }
    return false ;
}
 bool chmax (int a,int b){
    if ( a < b ){
    a = b;
    return true ;
    }
    return false ;
}   
#define SORT(c) sort((c).begin(),(c).end());
#define ANS(ans) cout<<(ans)<<endl;
 string split (string s , int  n , char x){
   // n文字目は前の文字列
   string a ;
   if ( x == 'f'){
     return a  = s.back(n) ;
   }else ( x == 'l'){
     return a = s.substr(n,s.size()-1) ;
   }
 }
/*CODE START HERE*/
int main() {
  string s;
  cin >> s;
  forn(i,n){
    s = s.back();
    if ( s.size()%2 == 1){
      continue;
    }
    string a = split ( s ,s.size()/2-1,f);
    string b = split ( s ,s.size()/2-1,f);
    if ( a == b){
      ANS(s.size());
      return 0;
    }
  }
}

Compiler output:
a.cc: In function 'std::string split(std::string, int, char)':
a.cc:86:24: error: no matching function for call to 'std::__cxx11::basic_string<char>::back(int&)'
   86 |      return a  = s.back(n) ;
      |                  ~~~~~~^~~
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.h:1350:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::back() [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; reference = char&]'
 1350 |       back() noexcept
      |       ^~~~
/usr/include/c++/14/bits/basic_string.h:1350:7: note:   candidate expects 0 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:1362:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::const_reference std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::back() const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; const_reference = const char&]'
 1362 |       back() const noexcept
      |       ^~~~
/usr/include/c++/14/bits/basic_string.h:1362:7: note:   candidate expects 0 arguments, 1 provided
a.cc:87:21: error: expected ';' before '{' token
   87 |    }else ( x == 'l'){
      |                     ^
      |                     ;
a.cc: In function 'int main()':
a.cc:95:10: error: 'n' was not declared in this scope
   95 |   forn(i,n){
      |          ^
a.cc:47:41: note: in definition of macro 'forr'
   47 | #define forr(i,a,b) ; for(int i=(a); i<(b); i++)
      |                                         ^
a.cc:95:3: note: in expansion of macro 'forn'
   95 |   forn(i,n){
      |   ^~~~
a.cc:100:40: error: 'f' was not declared in this scope
  100 |     string a = split ( s ,s.size()/2-1,f);
      |                                        ^