use strict;
use warnings;

my %thingy;

while (<DATA>) {
   chomp;
   my ($index,$value) = split;
   push @{$thingy{$index}}, $value;
}

for (sort keys %thingy) {
   print "$_ ", join('-',@{$thingy{$_}}), "\n";
}

__DATA__
1 0
1 1
2 0
3 1 
4 3
4 6