#include<stdio.h>
int main()
   int score;
   scanf("%d",&score);
   if(score<0 || score>100)
     printf("Invalid input");
   else{
   char grade=(score>=90)?'A':
            (score>=80)?'B':
            (score>=70)?'c':
            (score>=60)?'D':'F';
    printf("%c",grade);
    }
    return 0:
}