#include <stdio.h>

int main() {
    int n1, n2, n3;
    scanf ("%d %d %d", &n1, &n2, &n3);
    int count = 0;
    float avg = (n1 + n2 + n3) / 3.00;
    int flag = 0; 
    if (n1 < 70) {
        count++;
    }
    if (n2 < 70) {
        count++;
    }
    if (n3 < 70) {
        count++;
    }
    if (count >= 2) {
        if(n1 != 100 && n2 != 100 && n3 != 100){
            printf("F\n");
            flag = 1; 
        }
    }
    if (flag == 0){
        if (avg >= 85) {
            printf("A\n");
        }
        else if (avg >= 70 && avg < 85) {
            printf("B\n");
        }
        else {
            printf("F\n");
        }
    }
    printf("%.2f", avg);
    return 0;
}