10 INPUT "Enter a number: ";n
20 LET t=0
30 GO SUB 60
40 PRINT t
50 STOP
60 LET nold1=1: LET nold2=0
70 IF n<0 THEN PRINT "Positive argument required!": RETURN
80 IF n=0 THEN LET t=nold2: RETURN
90 IF n=1 THEN LET t=nold1: RETURN
100 LET t=nold2+nold1
110 IF n>2 THEN LET n=n-1: LET nold2=nold1: LET nold1=t: GO SUB 100
120 RETURN