<?php

echo strangeCounter(4)."\n";
echo strangeCounter(10)."\n";
echo strangeCounter(21)."\n";

function strangeCounter($t){
	$scnt=3;

while ($t+2>=$scnt){
		$scnt*=2;
	}
	// The sum of the number pairs inside the block is 
    // always the same. The sum is less by 2 than 2*$scnt
    return $scnt-$t-2; 
}