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