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;
 
/*CODE START HERE*/
int main() {
  string s;
  cin >> s;
  int ans =0
  forn(i,(s,size()-1)/2){
    bool a = true;
    forn ( j ,i){
      if ( s.at(j)!=a.at(j+i)){
        a = false;
        break ;
      }
      if(!a){
        break;
      }else if(a){
        ans = i;
    }
  }
    cout << ans*2 << endl;
    }

Compiler output:
a.cc: In function 'int main()':
a.cc:47:21: error: expected ',' or ';' before 'for'
   47 | #define forr(i,a,b) for(int i=(a); i<(b); i++)
      |                     ^~~
a.cc:49:19: note: in expansion of macro 'forr'
   49 | #define forn(i,n) forr(i,0,n)
      |                   ^~~~
a.cc:60:3: note: in expansion of macro 'forn'
   60 |   forn(i,(s,size()-1)/2){
      |   ^~~~
a.cc:60:8: error: 'i' was not declared in this scope
   60 |   forn(i,(s,size()-1)/2){
      |        ^
a.cc:47:36: note: in definition of macro 'forr'
   47 | #define forr(i,a,b) for(int i=(a); i<(b); i++)
      |                                    ^
a.cc:60:3: note: in expansion of macro 'forn'
   60 |   forn(i,(s,size()-1)/2){
      |   ^~~~
a.cc:60:17: error: no matching function for call to 'size()'
   60 |   forn(i,(s,size()-1)/2){
      |             ~~~~^~
a.cc:47:39: note: in definition of macro 'forr'
   47 | #define forr(i,a,b) for(int i=(a); i<(b); i++)
      |                                       ^
a.cc:60:3: note: in expansion of macro 'forn'
   60 |   forn(i,(s,size()-1)/2){
      |   ^~~~
In file included from /usr/include/c++/14/string:53,
                 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/range_access.h:262:5: note: candidate: 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/range_access.h:272:5: note: candidate: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note:   candidate expects 1 argument, 0 provided
a.cc:74:6: error: expected '}' at end of input
   74 |     }
      |      ^
a.cc:56:12: note: to match this '{'
   56 | int main() {
      |            ^