use strict;
use warnings;

my @grade_lookup = reverse(
   [ 90 => "A" ], 
   [ 80 => "B" ], 
   [ 70 => "C" ], 
   [ 60 => "D" ], 
);

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

my $grade = "F";
foreach (@grade_lookup) {
   if ( $score >= $_->[0] ) {
      $grade = $_->[1];
   }
}

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