import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int m=0,f=0,r=0,sum;
        String s="F";
        while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        }
    }
}
 import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int m=0,f=0,r=0,sum;
        String s="F";
        while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        }
    }
} Main public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int m=0,f=0,r=0,sum;
        String s="F";
        while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        }
    } main {
        Scanner scan=new Scanner(System.in);
        int m=0,f=0,r=0,sum;
        String s="F";
        while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        }
    } Scanner scan=new Scanner(System.in); scan new Scanner(System.in) int m=0 m 0 f=0 f 0 r=0 r 0 sum; sum String s="F"; s "F" while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        } true {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        } m=scan.nextInt() m scan.nextInt() scan.nextInt scan f=scan.nextInt() f scan.nextInt() scan.nextInt scan r=scan.nextInt() r scan.nextInt() scan.nextInt scan if(m==-1 && f==-1 && r==-1) {
        		break;
        	} m==-1 && f==-1 && r==-1 m==-1 && f==-1 && r==-1 m==-1 m -1 1 f==-1 f -1 1 r==-1 r -1 1 {
        		break;
        	} break; sum=m+f sum m+f m f if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	} m==-1 || f==-1 || sum<30 m==-1 || f==-1 || sum<30 m==-1 m -1 1 f==-1 f -1 1 sum<30 sum 30 {
        		System.out.println("F");
        	} System.out.println("F") System.out.println System.out System System.out "F" if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	} sum>=80 sum 80 {
        		System.out.println("A");
        	} System.out.println("A") System.out.println System.out System System.out "A" if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	} sum<80 && sum>=65 sum<80 sum 80 sum>=65 sum 65 {
        		System.out.println("B");
        	} System.out.println("B") System.out.println System.out System System.out "B" if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	} sum<65 && sum>=50 sum<65 sum 65 sum>=50 sum 50 {
        		System.out.println("C");
        	} System.out.println("C") System.out.println System.out System System.out "C" if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	} sum<50 && sum>=30 sum<50 sum 50 sum>=30 sum 30 {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	} if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		} r>=50 r 50 {
        			System.out.println("C");
        		} System.out.println("C") System.out.println System.out System System.out "C" {
        			System.out.println("D");
        		} System.out.println("D") System.out.println System.out System System.out "D" String[] args args public class Main {

    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int m=0,f=0,r=0,sum;
        String s="F";
        while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        }
    }
} public class Main {

    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int m=0,f=0,r=0,sum;
        String s="F";
        while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        }
    }
} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scan=new Scanner(System.in);
        int m=0,f=0,r=0,sum;
        String s="F";
        while(true) {
        	m=scan.nextInt();
        	f=scan.nextInt();
        	r=scan.nextInt();
        	if(m==-1 && f==-1 && r==-1) {
        		break;
        	}
        	sum=m+f;
        	if(m==-1 || f==-1 || sum<30) {
        		System.out.println("F");
        	}
        	else if(sum>=80) {
        		System.out.println("A");
        	}
        	else if(sum<80 && sum>=65) {
        		System.out.println("B");
        	}
        	else if(sum<65 && sum>=50) {
        		System.out.println("C");
        	}
        	else if(sum<50 && sum>=30) {
        		if(r>=50) {
        			System.out.println("C");
        		}
        		else{
        			System.out.println("D");
        		}
        	}
        }
    }
}

