function goForward() {
	var letztesBild = 38; /* Hier einfach die NUMMER des letzten Bildes eintragen! */
	
	var image1 = document.getElementById('seite1');	
	var number1 = parseInt(getImageNumber(image1.src)) + 2;
	
	if (number1 < letztesBild) {
		image1.src = "pages/" + number1 + ".jpg";
	}
	else {
		return false;
	}
	
	var image2 = document.getElementById('seite2');
	var number2 = parseInt(getImageNumber(image2.src)) + 2;
	
	if (number2 <= letztesBild) {
		image2.src = "pages/" + number2 + ".jpg";
	}
	else {
		image2.src = "pages/dummy.jpg";
	}	
}

function goBackwards() {
	var image1 = document.getElementById('seite1');	
	var number1 = parseInt(getImageNumber(image1.src)) - 2;
	var image2 = document.getElementById('seite2');
	var number2 = parseInt(getImageNumber(image2.src)) - 2;
	
	if (number1 > 0) {
		image1.src = "pages/" + number1 + ".jpg";
		image2.src = "pages/" + number2 + ".jpg";
	}
	else {
		return false;
	}
}

function getImageNumber(strBild) {

	var start = strBild.length - 6;
	var ende  = strBild.substr(start, 2);

	if(isNaN(ende)) {
		var start = strBild.length - 5;
		var ende = strBild.substr(start,1);
	}

	return ende;
}