Given the data is ordered as you show, this should do what you want:

my %data;

while (<DATA>) {
    chomp($_);
    my ($first, $second) = split(/\s+/, $_);
    push(@{$data{$first}}, $second);
}

foreach my $first (sort {$a <=> $b} keys %data) {
    print "$first ", join('-', @{$data{$first}}), "\n";
}
[download]

Hope this helps, -gjb-