$;=$";
@d=(-2,-2,-1,+1,+2,+2,+1,-1);
$n=<>;
for(0..$n-1){
	($x[$_],$y[$_])=($x,$y)=glob<>;
	$p{$x,$y}=1<<$_;
	for$j(0..7){
		$k{$x-$d[$j+6&7],$y-$d[$j]}=1;
	}
}
for(0..$n-1){
	delete $k{$x[$_],$y[$_]};
}
$b[0]=0;
for(keys%k){
	@a=@b;
	$k=$k{$_};
	($x,$y)=glob;
	for$j(0,2,4,6){
		$p=0;
		for(0,1){
			$p|=$p{$x+$d[$j+6&7^$_],$y+$d[$j^$_]};
		}
		if($p){
			for(0..(1<<$n)-1){
				if(defined$a[$_]){
					$b[$_|$p]=$a[$_]+1 if !defined $b[$_|$p] || $b[$_|$p]>$a[$_]+1;
				}
			}
		}
	}
}
print $b[(1<<$n)-1]//-1;