What is the smallest natural number that can not be generated using the numbers 1,2,3,4 (exactly once each) and four basic arithmetic operations?

gistfile1.py

import operator
from sys import maxint
from itertools import permutations, product

ops = [operator.add, operator.sub, operator.mul, operator.div]

possibilities = set(reduce(lambda v, t: t[0](v,t[1]), zip(o, n[1:]), n[0]) for n in permutations(range(1,5)) for o in product(ops, repeat=3))

# Without repeating operators:
# possibilities = set(reduce(lambda v, t: t[0](v,t[1]), zip(o, n[1:]), n[0]) for n in permutations(range(1,5)) for o in permutations(ops, repeat=3))

next(x for x in xrange(maxint) if x not in possibilities)