import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args)throws Exception{
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        
        while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        }
        
		
	}

} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {
	public static void main(String[] args)throws Exception{
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        
        while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        }
        
		
	}

} Main public static void main(String[] args)throws Exception{
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        
        while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        }
        
		
	} main {
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        
        while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        }
        
		
	} InputStreamReader isr = new InputStreamReader(System.in); isr new InputStreamReader(System.in) BufferedReader br = new BufferedReader(isr); br new BufferedReader(isr) while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        } true {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        } String[] test = br.readLine().split(" "); test br.readLine().split(" ") br.readLine().split br.readLine() br.readLine br " " int m = Integer.parseInt(test[0]); m Integer.parseInt(test[0]) Integer.parseInt Integer test[0] test 0 int f = Integer.parseInt(test[1]); f Integer.parseInt(test[1]) Integer.parseInt Integer test[1] test 1 int r = Integer.parseInt(test[2]); r Integer.parseInt(test[2]) Integer.parseInt Integer test[2] test 2 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; if ((m == -1) || (f == - 1))
            	System.out.println("F");
            else
            {
            	if((m + f) >= 80)
                	System.out.println("A");
            	else if((m + f) >= 65)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            } if((m + f) >= 80)
                	System.out.println("A");
            	else if((m + f) >= 65)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F"); (m + f) >= 65 (m + f) m f 65 System.out.println("B") System.out.println System.out System System.out "B" if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F"); (m + f) >= 50 (m + f) m f 50 System.out.println("C") System.out.println System.out System System.out "C" if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F"); (m + f) >= 30 (m + f) m f 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 System.out System System.out "C" 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 public class Main {
	public static void main(String[] args)throws Exception{
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        
        while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        }
        
		
	}

} public class Main {
	public static void main(String[] args)throws Exception{
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        
        while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        }
        
		
	}

} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args)throws Exception{
		InputStreamReader isr = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(isr);
        
        while (true)
        {
        	String[] test = br.readLine().split(" ");
            int m = Integer.parseInt(test[0]);
            int f = Integer.parseInt(test[1]);
            int r = Integer.parseInt(test[2]);
            if(m == -1 && f == -1 && r == -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)
                	System.out.println("B");
            	else if((m + f) >= 50)
                	System.out.println("C");
            	else if((m + f) >= 30)
            	{
            		if (r >= 50)
            			System.out.println("C");
            		else
            			System.out.println("D");
            	}
            	else if ((m + f) < 30)
                	System.out.println("F");
            }
            
        }
        
		
	}

}