sub isprime {
    my $n=shift;
    return undef if abs $n == 1;
    return isprime -$n if $n<0;
    return 0 if $n == 0;
    $n%$_ or return 0 for (2..sqrt $n);
    1;
}

##</code><code>##

sub isprime {
    (1 x shift) !~ /^1?$|^(11+?)\1+$/;
}

##</code><code>##

sub nearest_prime {
    my $n=my $m=shift;
    return $n if isprime $n;
    while (1) {
        return $n if isprime ++$n;
        return $m if isprime --$m;
    }
}