100 cls
110 sub fib(num)
120   if num < 0 then print "Invalid argument: "; : fib = num
130   if num < 2 then fib = num else fib = fib(num-1)+fib(num-2)
140 end sub
190 print fib(20)
200 print fib(30)
210 print fib(-10)
220 print fib(10)
230 end