def F_iter(n):
    #count_iter = count_iter + 1
    if (n == 0):
        return 0
    elif (n == 1):
        return 1
    elif (n >1 ):
        fn = 0
        fn1 = 1
        fn2 = 2
        for i in range(3, n):
            fn = fn1+fn2
            fn1 = fn2
            fn2 = fn
        return fn
    else:
        return -1


def F(n):
    #counter_recurs  = counter_recurs+ 1
    if (n == 0):
        return 0
    elif (n == 1):
        return 1
    elif (n > 1):
        return (F(n-1) + F(n-2))
    else:
        return -1