Language: C++

#include<bits/stdc++.h>

int main(){
	int n;
  	cin >> n;
  	if(n<600) cout << "8級\n";
  	else if(n<800) cout << "7級\n";
  	else if(n<1000) cout << "6級\n";
  	else if(n<1200) cout << "5級\n";
  	else if(n<1400) cout << "4級\n";
  	else if(n<1600) cout << "3級\n";
  	else if(n<1800) cout << "2級\n";
  	else cout << "1級\n";
  
  	return 0;

}

Compiler output:
a.cc: In function 'int main()':
a.cc:5:9: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    5 |         cin >> n;
      |         ^~~
      |         std::cin
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:146,
                 from a.cc:1:
/usr/include/c++/14/iostream:62:18: note: 'std::cin' declared here
   62 |   extern istream cin;           ///< Linked to standard input
      |                  ^~~
a.cc:6:19: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    6 |         if(n<600) cout << "8級\n";
      |                   ^~~~
      |                   std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:7:24: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    7 |         else if(n<800) cout << "7級\n";
      |                        ^~~~
      |                        std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:8:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    8 |         else if(n<1000) cout << "6級\n";
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:9:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    9 |         else if(n<1200) cout << "5級\n";
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:10:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   10 |         else if(n<1400) cout << "4級\n";
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:11:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   11 |         else if(n<1600) cout << "3級\n";
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:12:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   12 |         else if(n<1800) cout << "2級\n";
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:13:14: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   13 |         else cout << "1級\n";
      |              ^~~~
      |              std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~