function checkInput() {
	var error = "";
	
	if (document.getElementById("firstname").value == "") {
		error += "- Invalid first name.\n";
	}
	
	if (document.getElementById("secondname").value == "") {
		error += "- Invalid second name.\n";
	}
	
	if (document.getElementById("company").value == "") {
		error += "- Invalid company.\n";
	}
	
	if (document.getElementById("zipcode").value == "") {
		error += "- Invalid zipcode.\n";
	}
	
	if (!isValidEmail(document.getElementById("email").value)) {
		error += "- Invalid email.\n";
	}
	
	if (error != "") {
		alert(error + "\nPlease, check your input.");
		return false;
	}
	
	return true;
}

function isValidEmail(str)
{
	var regexp = /^[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-_]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-_]*[a-z0-9])?$/gi;
	return regexp.test(str);
}