#! perl -slw
use strict;
use 5.10.0;

printf "%s", scalar <DATA>;
my @points = map[ split ' ', $_ ], <DATA>;

my @modified;
push @modified, shift @points; # Kickstart

while ( @points ) {
    my $cmp = ($points[0][0] <=> $modified[-1][0])*($points[0][1] > $modified[-1][1]);
    if ($cmp == 1) {
        push @modified, shift @points;
    } elsif ($cmp == -1) {
        shift @points;
    } else {
        pop @modified;
    }
}