#include <stdio.h>

int main()
{
    int numgrade;
    scanf("%d", &numgrade);
    if (numgrade >= 80)
	printf("A\n");
    else if (numgrade >= 70)
	printf("B\n");
    else if (numgrade >= 60)
	printf("C\n");
    else if (numgrade >= 50)
	printf("D\n");
    else
	printf("F\n");
    return 0;
}