#include <stdio.h>
#include <assert.h>


int main()
{
    char grade;
    int score;
    
    scanf("%d", &score) ;

    if((score>=90) && (score<=100)) 
    {
        grade='a';
    } else if((score>=80) && (score<90))
    {
        grade='b';
    } else if((score>=70) && (score<80))
    {
        grade='c'; 
    } else if((score>=60) && (score<70))
    {
        grade='d' ;
    } else 
    {
        grade='f';
    }

       
    printf("%c \n", grade);
    //assert(grade=='c');
    return 0;

#include <stdio.h>
#include <assert.h>
int main()
{
	char grade;
	int score;
	
	scanf("%d", &score);
	if((100>=score) && (score>=90)){
		grade='a';
		}
	else if((90>score) && (score>=80)){
		grade='b';
		}
	else if((80>score) && (score>=70)){
		grade='c';
		}
	else if((70>score) && (score>=0)){
		grade='d';
		}
	else{grade='f';}


	printf("%c" , grade);


	//assert(grade=='c');

	return 0;
}