Millisecond Forums

Hi everyone,
I am struggling with the following challenge.
I have a survey page with four single textbox-questions where I ask participants to evaluate how much percent of their attention in the trial before was directed to four different categories. So, e.g., 20% to the task before, 70% to something else...
I would like to check that participants can only proceed to the next survey page if the total amount of their shares of thoughts equals 100%. If it doesent, there shall be a warning message and particpiants need to adjust there answerts. My problem is, that I do not know how to implement this check. I need to add all the answers and check, whether the sum of the four answers = 100 percent. If so, the next surverypage should follow, if not participants would be encouraged to adapt their choice of values.
I tried something like the following, but do not know how to proceed:

/ontrialend = [if ((textbox.Seiteabbruch.response) + (textbox.komplettanderegedanken.response)+
(textbox.NBackGedanken.response)+ (textbox.NBackPerformanceGedanken.response)) == 100]

I would be very happy about any help.
This is the code.

<textbox ApartmentGedanken>
/ caption = "... der Wohnungsaufgabe von davor? (in Prozent)"
/ multiline = false
/ required = true
/ maxchars = 3
/textboxsize = (3.5%,2.5%)
</textbox>

slider ApartmentGedanken>
/ caption = "... die Apartment-Aufgabe von davor nachgedacht?"
/ subcaption = "        "
/ labels = ("0%","10%", "20%", "30%", "40%", "50%","60%", "70%", "80&", "90%","100 %")
/ range = (1%, 100%)
/ slidersize = (50%, 5%)
/required = false
/ showtooltips = false
/ increment = 1
</slider>

<textbox KomplettAndereGedanken>
/ caption = "... etwas ganz anderem? (in Prozent)"
/ multiline = false
/ required = true
/ maxchars = 3
/textboxsize = (3.5%,2.5%)
</textbox>


<textbox NBackGedanken>
/ caption = "... der 3-Zurück-Aufgabe? (in Prozent)"
/ multiline = false
/ required = true
/ maxchars = 3
/textboxsize = (3.5%,2.5%)
</textbox>


<surveypage Gedanken>
/caption ="XYZ."
/ subcaption = "XYZ."
/questions = [1 = ApartmentGedanken, KomplettAndereGedanken, NBackGedanken, NBackPerformanceGedanken]
/showpagenumbers = false
/showquestionnumbers = false
/ errormessage = true(text.error, 3000)
??????
</surveypage>