Language: C++

#include<bits/stdc++.h>

#define hell                         1000000007
#define lcm(a,b)                     (a*b)/__gcd(a,b)
#define ll                           long long
#define vll                          vector<ll>
#define vi                           vector<int>
#define pll                          vector< pair<ll,ll> >
#define pb                           push_back
#define mp                           make_pair
#define all(v)                       v.begin(),v.end()
#define lbnd                         lower_bound
#define ubnd                         upper_bound
#define bs                           binary_search
#define F                            first
#define S                            second
#define rep(i,a,b)                   for(i=a;i<b;i++)
#define parr(a,n)                    for(ll it=0;it<n;it++) cout<<a[it]<<" ";cout<<endl;
#define pcont(a)                     for(auto it:a) cout<<it<<" ";cout<<endl;
#define ret(x)                       return cout<<x,0;
#define endl                           '\n'

/*constants*/
#define MAXN                         100005
#define PI                           3.14159265358979323846

/*debug*/
#define TRACE
#ifdef TRACE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
    template <typename Arg1>void __f(const char* name, Arg1&& arg1){ std::cout << name << " : " << arg1 << '\n';}
    template <typename Arg1, typename... Args>void __f(const char* names, Arg1&& arg1, Args&&... args){const char* comma = strchr(names + 1, ','); std::cout.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);}
#else
#define trace(...)
#endif

using namespace std;


int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);cout.tie(NULL);
	
	ll i,j,n,m,k,cnt=0,ans=0,t=1;
	cin>>n>>m;
	ret(max({a+b,a*b,a-b}))	
	
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:47:18: error: 'a' was not declared in this scope
   47 |         ret(max({a+b,a*b,a-b}))
      |                  ^
a.cc:20:51: note: in definition of macro 'ret'
   20 | #define ret(x)                       return cout<<x,0;
      |                                                   ^
a.cc:47:20: error: 'b' was not declared in this scope
   47 |         ret(max({a+b,a*b,a-b}))
      |                    ^
a.cc:20:51: note: in definition of macro 'ret'
   20 | #define ret(x)                       return cout<<x,0;
      |                                                   ^
a.cc:47:16: error: no matching function for call to 'max(<brace-enclosed initializer list>)'
   47 |         ret(max({a+b,a*b,a-b}))
      |             ~~~^~~~~~~~~~~~~~~
a.cc:20:51: note: in definition of macro 'ret'
   20 | #define ret(x)                       return cout<<x,0;
      |                                                   ^
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 1 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   candidate expects 2 arguments, 1 provided