Language: C++

#include<iostream>
using namespace std;

int main(){
	int m, f, r;
	cin >> m >> f >> r;
	while(1){
	if(m=-1 || f=-1){
	cout >> "F" >> "\n";
	}if else(m+f>=80){
	cout >> "A" >> "\n";
	}if else(m+f<=65 && m+f<80){
	cout >> "B" >> "\n";
	}if else(m+f<=50 && m+f<65){
	cout >> "C" >> "\n"
	}if else(m+f<=30 && m+f<50){
	cout >> "D"  >> "\n";
	}if else(m+f<=30 && m+f<50 && r<=50){
	cout >> "C" >> "\n";
	}if else(m+f<30){
	cout >> "F" >> "\n";
	}
	cout >> "\n";
	}
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:8:17: error: lvalue required as left operand of assignment
    8 |         if(m=-1 || f=-1){
      |              ~~~^~~~
a.cc:9:14: error: no match for 'operator>>' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'const char [2]')
    9 |         cout >> "F" >> "\n";
      |         ~~~~ ^~ ~~~
      |         |       |
      |         |       const char [2]
      |         std::ostream {aka std::basic_ostream<char>}
In file included from /usr/include/c++/14/string:55,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.tcc:835:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
  835 |     operator>>(basic_istream<_CharT, _Traits>& __in,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.tcc:835:5: note:   template argument deduction/substitution failed:
a.cc:9:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
    9 |         cout >> "F" >> "\n";
      |                 ^~~
In file included from /usr/include/c++/14/bits/memory_resource.h:38,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/cstddef:131:5: note: candidate: 'template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType> std::operator>>(byte, _IntegerType)'
  131 |     operator>>(byte __b, _IntegerType __shift) noexcept
      |     ^~~~~~~~
/usr/include/c++/14/cstddef:131:5: note:   template argument deduction/substitution failed:
a.cc:9:9: note:   cannot convert 'std::cout' (type 'std::ostream' {aka 'std::basic_ostream<char>'}) to type 'std::byte'
    9 |         cout >> "F" >> "\n";
      |         ^~~~
In file included from /usr/include/c++/14/istream:1109,
                 from /usr/include/c++/14/iostream:42:
/usr/include/c++/14/bits/istream.tcc:978:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT&)'
  978 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT& __c)
      |     ^~~~~~~~
/usr/include/c++/14/bits/istream.tcc:978:5: note:   template argument deduction/substitution failed:
a.cc:9:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
    9 |         cout >> "F" >> "\n";
      |                 ^~~
/usr/include/c++/14/istream:849:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char&)'
  849 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:849:5: note:   template argument deduction/substitution failed:
a.cc:9:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    9 |         cout >> "F" >> "\n";
      |                 ^~~
/usr/include/c++/14/istream:854:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char&)'
  854 |     operator>>(basic_istream<char, _Traits>& __in, signed char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:854:5: note:   template argument deduction/substitution failed:
a.cc:9:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    9 |         cout >> "F" >> "\n";
      |                 ^~~
/usr/include/c++/14/istream:896:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT*)'
  896 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:896:5: note:   template argument deduction/substitution failed:
a.cc:9:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
    9 |         cout >> "F" >> "\n";
      |                 ^~~
/usr/include/c++/14/istream:939:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char*)'
  939 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:939:5: note:   template argument deduction/substitution failed:
a.cc:9:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    9 |         cout >> "F" >> "\n";
      |                 ^~~
/usr/include/c++/14/istream:945:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char*)'
  945 |     operator>>(basic_istream<char, _Traits>& __in, signed char* __s)
      |     ^~~~~~~~
/usr/include/c++/14/istream:945:5: note:   template argument deduction/substitution failed:
a.cc:9:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
    9 |         cout >> "F" >> "\n";
      |                 ^~~
/usr/include/c++/14/istream:1099:5: note: candidate: 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&)'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~
/usr/include/c++/14/istream:1099:5: note:   template argument deduction/substitution failed:
/usr/include/c++/14/istream: In substitution of 'template<class _Istream, class _Tp> _Istream&& std::operator>>(_Istream&&, _Tp&&) [with _Istream = std::basic_ostream<char>&; _Tp = const char (&)[2]]':
a.cc:9:10:   required from here
    9 |         cout >> "F" >> "\n";
      |                 ^~~
/usr/include/c++/14/istream:1099:5: error: no type named 'type' in 'struct std::enable_if<false, void>'
 1099 |     operator>>(_Istream&& __is, _Tp&& __x)
      |     ^~~~~~~~
a.cc:10:13: error: expected '(' before 'else'
   10 |         }if else(m+f>=80){
      |             ^~~~
      |             (
a.cc:12:13: error: expected '(' before 'else'
   12 |         }if else(m+f<=65 && m+f<80){
      |             ^~~~
      |             (
a.cc:14:13: error: expected '(' before 'else'
   14 |         }if else(m+f<=50 && m+f<65){
      |             ^~~~
      |             (
a.cc:16:13: error: expected '(' before 'else'
   16 |         }if else(m+f<=30 && m+f<50){
      |             ^~~~
      |             (
a.cc:18:13: error: expected '(' before 'else'
   18 |         }if else(m+f<=30 && m+f<50 && r<=50){
      |             ^~~~
      |             (
a.cc:20:13: error: expected '(' before 'else'
   20 |         }if else(m+f<30){
      |             ^~~~
      |             (
a.cc:23:14: error: no match for 'operator>>' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'const char [2]')
   23 |         cout >> "\n";
      |         ~~~~ ^~ ~~~~
      |         |       |
      |         |       const char [2]
      |         std::ostream {aka std::basic_ostream<char>}
/usr/include/c++/14/bits/basic_string.tcc:835:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
  835 |     operator>>(basic_istream<_CharT, _Traits>& __in,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.tcc:835:5: note:   template argument deduction/substitution failed:
a.cc:23:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   23 |         cout >> "\n";
      |                 ^~~~
/usr/include/c++/14/cstddef:131:5: note: candidate: 'template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType> std::operator>>(byte, _IntegerType)'
  131 |     operator>>(byte __b, _IntegerType __shift) noexcept
      |     ^~~~~~~~
/usr/include/c++/14/cstddef:131:5: note:   template argument deduction/substitution failed:
a.cc:23:9: note:   cannot convert 'std::cout' (type 'std::ostream' {aka 'std::basic_ostream<char>'}) to type 'std::byte'
   23 |         cout >> "\n";
      |         ^~~~
/usr/include/c++/14/bits/istream.tcc:978:5: note: candidate: 'template<class _CharT, class _Traits> std::basic_istream<_CharT, _Traits>& std::operator>>(basic_istream<_CharT, _Traits>&, _CharT&)'
  978 |     operator>>(basic_istream<_CharT, _Traits>& __in, _CharT& __c)
      |     ^~~~~~~~
/usr/include/c++/14/bits/istream.tcc:978:5: note:   template argument deduction/substitution failed:
a.cc:23:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<_CharT, _Traits>'
   23 |         cout >> "\n";
      |                 ^~~~
/usr/include/c++/14/istream:849:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, unsigned char&)'
  849 |     operator>>(basic_istream<char, _Traits>& __in, unsigned char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:849:5: note:   template argument deduction/substitution failed:
a.cc:23:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::basic_istream<char, _Traits>'
   23 |         cout >> "\n";
      |                 ^~~~
/usr/include/c++/14/istream:854:5: note: candidate: 'template<class _Traits> std::basic_istream<char, _Traits>& std::operator>>(basic_istream<char, _Traits>&, signed char&)'
  854 |     operator>>(basic_istream<char, _Traits>& __in, signed char& __c)
      |     ^~~~~~~~
/usr/include/c++/14/istream:854:5: note:   template argument deduction/substitution failed:
a.cc:23:17: note:   'std::ostream' {aka 'std::basic_ostream<char>'} is not derived from 'std::ba