import java.util.ArrayList;
import java.util.Scanner;
public class Grading {
private static final int NOT_COME_EXAM = -1;
private static Scanner sc = new Scanner(System.in);
private static ArrayList<String> result = new ArrayList<String>();
public static void main(String[] args) {
gradingAll();
printAllGrades();
}
private static String getGrade(int m, int f, int r){
int sum = m + f;
if( m == NOT_COME_EXAM || f == NOT_COME_EXAM){
return "F";
}
else if(sum >= 80){
return "A";
}
else if(sum >= 65){
return "B";
}
else if(sum >= 50){
return "C";
}
else if(sum >= 30){
if(r >= 50){
return "C";
}
else{
return "D";
}
}
else if(sum < 30){
return "F";
}
return "F";
}
private static void grading(int m, int f, int r){
result.add(getGrade(m, f, r));
}
private static void gradingAll(){
int m = -1;
int f = -1;
int r = -1;
while(sc.hasNextLine()){
m = sc.nextInt();
f = sc.nextInt();
r = sc.nextInt();
if((m == -1) && (f == -1) && (r == -1)){
break;
}
else{
grading(m, f, r);
}
}
}
private static void printAllGrades(){
for(int i = 0; i < result.size(); i++){
System.out.println(result.get(i));
}
}
}