def a12toi(s):
	if not s:
		return 0
	i = 0
	sign = 1
	s0 = s[:1]
	if s0 in ('-', '+'):
		s = s[1:]
	if s0 == '-':
		sign = -1

	digits = "0123456789ab"

	try:
		for c in s:
			i = i * 12 + digits.index(c)
	except ValueError:
		pass
		
	return sign * i

def test():
	assert(a12toi("") == 0);
	assert(a12toi("b") == 11);
	assert(a12toi("10c") == 12);
	assert(a12toi("+123ab") == 24755);
	assert(a12toi("-123ab") == -24755);

test()