Language: C

#includ <stdio.h>
main(void){
   int m,f,r;
   int sum;
   m = -2;
   for(m==-1&&f==-1&&r==-1){
      scanf("%d%d%d",&m,&f,&r);
      if(m==-1&&f==-1)printf("%s\n","F");
      sum = m + f;
      if(sum>=80)printf("%s\n","A");
      else if(sum>=65)printf("%s\n","B");
      else if(sum>=50)printf("%s\n","C");
      else if(sum>=30){
         if(r>=50)printf("%s\n","D");
         else printf("%s\n","F");
      }
      else printf("%s\n","F");
   }
   return 0;
}

Compiler output:
main.c:1:2: error: invalid preprocessing directive #includ; did you mean #include?
    1 | #includ <stdio.h>
      |  ^~~~~~
      |  include
main.c:2:1: error: return type defaults to 'int' [-Wimplicit-int]
    2 | main(void){
      | ^~~~
main.c: In function 'main':
main.c:6:27: error: expected ';' before ')' token
    6 |    for(m==-1&&f==-1&&r==-1){
      |                           ^
      |                           ;
main.c:6:27: error: expected expression before ')' token
main.c:7:7: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    7 |       scanf("%d%d%d",&m,&f,&r);
      |       ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | #includ <stdio.h>
main.c:7:7: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    7 |       scanf("%d%d%d",&m,&f,&r);
      |       ^~~~~
main.c:7:7: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:8:23: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    8 |       if(m==-1&&f==-1)printf("%s\n","F");
      |                       ^~~~~~
main.c:8:23: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:8:23: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:8:23: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:10:18: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   10 |       if(sum>=80)printf("%s\n","A");
      |                  ^~~~~~
main.c:10:18: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:11:23: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   11 |       else if(sum>=65)printf("%s\n","B");
      |                       ^~~~~~
main.c:11:23: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:12:23: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   12 |       else if(sum>=50)printf("%s\n","C");
      |                       ^~~~~~
main.c:12:23: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:14:19: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   14 |          if(r>=50)printf("%s\n","D");
      |                   ^~~~~~
main.c:14:19: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:15:15: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   15 |          else printf("%s\n","F");
      |               ^~~~~~
main.c:15:15: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:17:12: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   17 |       else printf("%s\n","F");
      |            ^~~~~~
main.c:17:12: note: include '<stdio.h>' or provide a declaration of 'printf'