sub reduce {
    my $exp = $_[0];
    1 while $exp =~ s{(\d+)\s+([*/])\s+(\d+)}
                     { $2 eq '*' ? $1*$3 : $1/$3 }e;
    1 while $exp =~ s{(\d+)\s+([+-])\s+(\d+)}
                     { $2 eq '+' ? $1+$3 : $1-$3 }e;
    return $exp;
}