use Getopt::Std;
my %opts;

die "Usage $0 -m or -M #1 #2 #3..." if (!getopts( "mM", \%opts)
    || @ARGV !=0 || keys %opts != 2);
CalculateMean()   if $opts{m}; #@ARGV has just #1 #2 #3 ...now.
CalculateMedian() if $opts{M};