import java.util.*;

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

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

