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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    }
  }
} import java.util.Scanner; Scanner java.util 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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    }
  }
} 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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    }
  } main { 
    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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    }
  } Scanner sc = new Scanner(System.in); 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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    } true {
      int m = sc.nextInt(); 
      int f = sc.nextInt();
      int r = sc.nextInt();
      if (m == -1 && f == -1 && r == -1){ 
        break; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    } int m = sc.nextInt(); m sc.nextInt() sc.nextInt sc int f = sc.nextInt(); f sc.nextInt() sc.nextInt sc int r = sc.nextInt(); r sc.nextInt() sc.nextInt sc 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; if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<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("F") System.out.println System.out System System.out "F" {
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      } int S = m + f; S m + f m f if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        } S>=80 S 80 {
          System.out.println("A");
        } System.out.println("A") System.out.println System.out System System.out "A" if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        } S<80 && S>=65 S<80 S 80 S>=65 S 65 {
          System.out.println("B");
        } System.out.println("B") System.out.println System.out System System.out "B" if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        } S<65 && S>=50 S<65 S 65 S>=50 S 50 {
          System.out.println("C");
        } System.out.println("C") System.out.println System.out System System.out "C" if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        } S<50 && S>=30 S<50 S 50 S>=30 S 30 {
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        } if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            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" if(r<50){
            System.out.println("D");
          } r<50 r 50 {
            System.out.println("D");
          } System.out.println("D") System.out.println System.out System System.out "D" if(S<30){
          System.out.println("F");
        } S<30 S 30 {
          System.out.println("F");
        } 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) { 
    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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    }
  }
} 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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    }
  }
} Main
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; 
      } 
      if(m==-1 || f==-1){
        System.out.println("F");
      } 
      else{
        int S = m + f;
        if(S>=80){
          System.out.println("A");
        }
        else if(S<80 && S>=65){
          System.out.println("B");
        }
        else if(S<65 && S>=50){
          System.out.println("C");
        }
        else if(S<50 && S>=30){
          if(r>=50){
            System.out.println("C");
          }
          else if(r<50){
            System.out.println("D");
          }
        }
        else if(S<30){
          System.out.println("F");
        }
      }
    }
  }
}