step with a file read variable

Hey there. I can't seem to get blitz to assign a variable read from a file to a const so that I can use it with a for-step-next loop.

a$ = Input$("Specify Source:")
Global img=LoadImage(a$+".jpg") 
Global file$ = a$ + ".txt"
Global linkfile = ReadFile(file$)

Const steps = ReadInt (linkfile)

For y = 0 To ImageHeight(img) Step steps
	WriteLine( fileout, "..."  )
	For x = 0 To ImageWidth(img) Step steps
		If y <= ImageHeight(img)
			If x <= ImageWidth(img)
				WriteLine( fileout, "...stuff..."  )
			EndIf
		EndIf	
	Next
	WriteLine( fileout, "...."  )
Next

It keeps saying "expression must be constant"
Is there any way I can use a file variable to change the step amount?