#include <stdio.h>

int main() {
	int x;
	scanf("%d",&x);
	if ( x>=90 )
		printf(" S\n");
	else if (  90>x>=80  )
		printf("A \n");
	else if ( 80>x>=60)
		printf("B\n");
	else
		printf("D\n");
	return 0;
}