#include <stdio.h>

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