FUNCTION itFib (n)
n1 = 0
n2 = 1
FOR k = 1 TO ABS(n)
sum = n1 + n2
n1 = n2
n2 = sum
NEXT k
IF n < 0 THEN
itFib = n1 * ((-1) ^ ((-n) + 1))
ELSE
itFib = n1
END IF
END FUNCTION