chomp (my $grade = <STDIN>);
$grade !~ /^\d+$/ and die "Input must be numeric!\n";

# hash with upper threshold as keys
my %scores = (
    90 => 'A',
    80 => 'B',
    70 => 'C',
    60 => 'D',
    50 => 'F',
);
my $result;
foreach my $key (sort keys %scores) {
    if ($grade <= $key) {
        $result = $scores{$key};
        last;
    }
}
print $result, $/;