#! 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;
    }
}

##</code><code>##

while ( @points ) {
    if (($points[0][0] > $modified[-1][0])^($points[0][1] < $modified[-1][1])) {
        push @modified, shift @points;
    } else {
        shift @points;
        pop @modified;
    }
}

##</code><code>##

while ( @points ) {
    if (($points[0][0]-$modified[-1][0])/($points[0][1]-$modified[-1][1]) > 0) {
        push @modified, shift @points;
    } else { # Retrograde!
        my @retro = (pop @modified, shift @points);
        push @retro, shift @points while ($retro[-1][0]-$points[0][0])/($retro[-1][1]-$points[0][1]) < 0;
        shift @points while ($retro[0][0]-$points[0][0])/($retro[0][1]-$points[0][1]) < 0;
        pop @modified while ($retro[-1][0]-$modified[-1][0])/($retro[-1][1]-$modified[-1][1]) < 0;
        push @modified, [0.5*($retro[@retro/2-.5][0] + $retro[@retro/2][0]),
                         0.5*($retro[@retro/2-.5][1] + $retro[@retro/2][1]),
                        ]
    }
}