function ch_bx(n, a) { //change boxes(num of boxes, actual)
	
	if (!n) return(false);
	if (!a) a = 1;

	for (var i = 1; i <= n; i++) {
		
		document.getElementById('t' + i).style.display = 'none';
	}

	document.getElementById('t' + a).style.display = 'block';
	
}