import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

while (true) {
int m = sc.nextInt();
int f = sc.nextInt();
int r = sc.nextInt();

if (m == -1 && f == -1 && r == -1) {
    break;
} else if (m == -1 || f == -1){
    System.out.println("F");
} else if(m+f>=80){
    System.out.println("A");
} else if(65<=m+f && m+f<80){
    System.out.println("B");
} else if(50<=m+f && m+f<65){
    System.out.println("C");
} else if(30<=m+f && m+f<50 && r<50){
    System.out.println("D");
} else if(30<=m+f && m+f<50 && r>=50){
    System.out.println("C");
} else {
    System.out.println("F");
}
    }
}
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz