def exec_time(func):
    def inner(*args, **kwargs):
        import time
        st = time.time()
        ret = func(*args, **kwargs)
        et = time.time() - st
        print "execution time %s seconds" %(et)
        return ret

    return inner

@exec_time
def fabonacci(n):
    if n == 0:
        return 0
    elif n == 1:
        return 1

    a, b = 0, 1
    i = 2
    while i < n:
        a, b = b, a + b
        i += 1

    return b

if __name__ == "__main__":
    print fabonacci(20)