#include <stdio.h>

 int main()

 {

     int grade;

     scanf_s("%d", &grade);

     grade = grade / ;

     switch (grade)

     {

         case :

         case :printf("A\n"); break;

         case :printf("B\n"); break;

         case :printf("C\n"); break;

         case :printf("D\n"); break;

         default:printf("F\n");break;

     }

     return ;

 }