chomp (my $grade = <STDIN>);
$grade !~ /^\d+$/ and die "Input must be numeric!\n";
my %scores = (
    A => sub { return $_[0] <= 90 ? 1 : 0 },
    B => sub { return $_[0] <= 80 ? 1 : 0 },
    C => sub { return $_[0] <= 70 ? 1 : 0 },
    D => sub { return $_[0] <= 60 ? 1 : 0 }, # just a guess, otherwise like 'B'
    F => sub { return $_[0] >= 50 ? 1 : 0 }, # really? shouldn't that be '<=' ?
);
my $result;
foreach my $key (qw(D C B A F)) {
    if ($scores{$key}->($grade) ) {
        $result = $key;
        last;
    }
}
print $result,$/;