use strict;
use warnings;

my %grade_lookup = (
   90 => "A", 
   80 => "B", 
   70 => "C", 
   60 => "D", 
);

my $score = <STDIN>;
chomp($score);

my $grade = "F";
foreach my $gate_score (sort { $a <=> $b } keys %grade_lookup) {
   if ( $score >= $gate_score ) {
      $grade = $grade_lookup{$gate_score};
   }
}

print("$score is $grade\n");