#include <stdio.h>
int main(void)
{
    int percentage;
    char grade;
    if (scanf("%d", &percentage) != 1 || percentage < 0 || percentage > 100) {
        puts("Enter a whole-number percentage from 0 to 100.");
        return 1;
    }
    switch (percentage / 10) {
        case 10:
        case 9:
        case 8: grade = 'A'; break;
        case 7:
        case 6: grade = 'B'; break;
        case 5:
        case 4: grade = 'C'; break;
        default: grade = 'F';
    }
    printf("Grade = %c\n", grade);
    return 0;
}