#include <stdio.h>

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

    switch(score/10){
        case 10:
        case 9:
            printf("A");
            break;
        case 8:
            printf("B");
            break;
        case 7:
            printf("C");
            break;
        default:
            printf("F")
            break;
    }
}