Problem with negative numbers for INPUT function

[UncleJohn]

I have just registered so I apologize for not being familiar with your procedures. I purchased a wireless Prime directly from HP late December. Currently I am having a problem with a simple Mandelbrot program . I use INPUT to enter data including the x and y centres for the region of interest. I supply default values one of which is negative . If I run the program without editing the negative value , it runs as expected . if I enter any negative value ,including an exact copy of the negative default, then an Invalid Input error message is displayed. I will attach a copy of the program. I would be pleased to hear from anyone who can point out what is causing this behaviour. I should note that I have experienced difficulty obtaining documentation on the Prime,for example I have not found an example of using INPUT for multiple lines of data .

Thanks in advance

UncleJohn
[attachment=3130][attachment=3130][attachment=3130][attachment=3130][attachment=3130][attachment=3130]