Language: C++

#include <bits/stdc++.h>

int main(){
	int m,f,r;

	for(;;){
		cin>>m>>f>>r;
		if(m == f&&f == r&&m == 0)
			break;

		if(m == -1 ||f == -1)
			cout<<"F"<<endl;

		else if(m+f>=80)
			cout<<"A"<<endl;

		else if(m+f>=65&&m+f<80)
			cout<<"B"<<endl;

		else if(m+f>=50&&m+f<65)
			cout<<"C"<<endl;

		else if(m+f>=30&&m+f<50){
			if(r >= 50)
				cout<<"C"<<endl;
			else
				cout<<"D"<<endl;
		}
		else
			cout<<"F"<<endl;

	}
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:7:17: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    7 |                 cin>>m>>f>>r;
      |                 ^~~
      |                 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:12:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   12 |                         cout<<"F"<<endl;
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:12:36: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   12 |                         cout<<"F"<<endl;
      |                                    ^~~~
      |                                    std::endl
In file included from /usr/include/c++/14/istream:41,
                 from /usr/include/c++/14/sstream:40,
                 from /usr/include/c++/14/complex:45,
                 from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:15:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   15 |                         cout<<"A"<<endl;
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:15:36: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   15 |                         cout<<"A"<<endl;
      |                                    ^~~~
      |                                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:18:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   18 |                         cout<<"B"<<endl;
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:18:36: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   18 |                         cout<<"B"<<endl;
      |                                    ^~~~
      |                                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:21:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   21 |                         cout<<"C"<<endl;
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:21:36: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   21 |                         cout<<"C"<<endl;
      |                                    ^~~~
      |                                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:25:33: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   25 |                                 cout<<"C"<<endl;
      |                                 ^~~~
      |                                 std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:25:44: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   25 |                                 cout<<"C"<<endl;
      |                                            ^~~~
      |                                            std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:27:33: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   27 |                                 cout<<"D"<<endl;
      |                                 ^~~~
      |                                 std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:27:44: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   27 |                                 cout<<"D"<<endl;
      |                                            ^~~~
      |                                            std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
a.cc:30:25: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   30 |                         cout<<"F"<<endl;
      |                         ^~~~
      |                         std::cout
/usr/include/c++/14/iostream:63:18: note: 'std::cout' declared here
   63 |   extern ostream cout;          ///< Linked to standard output
      |                  ^~~~
a.cc:30:36: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   30 |                         cout<<"F"<<endl;
      |                                    ^~~~
      |                                    std::endl
/usr/include/c++/14/ostream:744:5: note: 'std::endl' declared here
  744 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~