Language: C++

/*input
9
*/
/*
FIND YOUR PURPOSE BECOME UNSTOPPABLE.
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            ,,-) ---._   |   .---''`-),.
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    _<    \     \ ,'  ') )   `. /     /    <,.
  ,-'   _,  \    ,'    ( /      `.    /        `-,
  `-.,-'     `.,'       `         `.,'  `\    ,-'
  ,'       _  /   ,,,      ,,,     \     `-. `-._
  /-,     ,'  ;   ' _ \    / _ `     ; `.     `(`-\
  /-,        ;    (o)      (o)      ;          `'`,
,~-'  ,-'    \     '        `      /     \      <_
/-. ,'        \                   /       \     ,-'
  '`,     ,'   `-/             \-' `.      `-. <
    /_    /      /   (_     _)   \    \          `,
      `-._;  ,' |  .::.`-.-' :..  |       `-.    _\
        _/       \  `:: ,^. :.:' / `.        \,-'
      '`.   ,-'  /`-..-'-.-`-..-'\            `-.
        >_ /     ;  (\/( ' )\/)  ;     `-.    _<
        ,-'      `.  \`-^^^-'/  ,'        \ _<
        `-,  ,'   `. `"""""' ,'   `-.   <`'
          ')        `._.,,_.'        \ ,-'
            '._        '`'`'   \       <
              >   ,'       ,   `-.   <`'
                `,/          \      ,-`
                `,   ,' |   /     /
                  '; /   ;        (
                  _)|   `       (
                  `')         .-'
                    <_   \   /    
                      \   /\(
                        `;/  `
*/
#include <bits/stdc++.h>
#define endl "\n"
#define ll long long int
#define vi vector<int>
#define vll vector<ll>
#define vvl vector < vll >
#define pii pair<int,int>
#define pll pair<long long, long long>
#define mod 1000000007
#define inf 1000000000000000001
#define all(c) c.begin(),c.end()
#define mem(a,val) memset(a,val,sizeof(a))
#define eb emplace_back
#define sz(a) int((a).size())
#define pb push_back
#define tr(c,i) for(typeof((c).begin() i = (c).begin(); i != (c).end(); i++)
#define present(c,x) ((c).find(x) != (c).end())
#define cpresent(c,x) (find(all(c),x) != (c).end())     
#define rep(i,j,n) for(ll i=j;i<n;i++)
#define mp make_pair
#define N 998244353
#define PI acos(-1.0)
//if (s.find('R') != std::string::npos)
//s.replace(s.find("10"), sizeof("10") - 1, "");
using namespace std;
ll gcd(ll a,ll b){
  if(b==0) return a;
  return gcd(b,a%b);
}
bool compare(const pair<int, int>&i, const pair<int, int>&j)
  {
    return i.second < j.second;
  }
bool isPrime(ll j)
{
  if(j==2)
    return true;
  if(j%2==0)
    return false;
  for(ll i=3;i*i<=j;i=i+2)
    if(j%i==0)
    return false;
  return true;
}
bool isSubSequence(string str1, string str2, int m, int n) 
{ 
  int j = 0; // For index of str1 (or subsequence 

  // Traverse str2 and str1, and compare current character  
  // of str2 with first unmatched char of str1, if matched  
  // then move ahead in str1 
  for (int i=0; i<n&&j<m; i++) 
    if (str1[j] == str2[i]) 
      j++; 

  // If all characters of str1 were found in str2 
  return (j==m); 
} 

long long  int exponentiation(long long  int base, long long  int exp) 
{ 
  if (exp == 0) 
    return 1; 

  if (exp == 1) 
    return base % N; 
  long long int t = exponentiation(base, exp / 2); 
  t = (t * t); 
  if (exp % 2 == 0) 
    return t;
  else  
    return ((base ) * t) ; 
} 
ll digit(ll n){
  ll c=0;
  while(n){
    n=n/10;
    c++;
  }
  return c;

}
ll countDigit(long long n) 
{ 
    return floor(log10(n) + 1); 
} 
bool can(ll m,ll s){
  return s >= 0 && s <= 9 * m;
}
int main()
{
  // added the two lines below 
  ios_base::sync_with_stdio (false); 
  cin.tie(NULL);
  cout.tie(0);
  #ifndef ONLINE_JUDGE
  
  ll a,b;
  cin>>a>>b;
  cout<<max(a+b,max(a*b,a-b));


  
  }

Compiler output:
a.cc:137: error: unterminated #ifndef
  137 |   #ifndef ONLINE_JUDGE