#include <stdio.h>

int main()
{
    int score = 0;
    scanf("%d", &score);
    int c = score / 10;
    if (score >= 0)
    {
        switch (c)
          {
              case 9:
              case 10:
                  printf("A\n");
                  break;
              case 8:
                  printf("B\n");
                  break;
              case 7:
                  printf("C\n");
                  break;
              case 6:
                  printf("D\n");
                  break;
              case 5:
              case 4:
              case 3:
              case 2:
              case 1:
              case 0:
                  printf("E\n");
                  break;
              default:
                  printf("F\n");
                  break;
          }
    }
    else
    {
        printf("F\n");
    }

    return 0;
}