NUMBERS = ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9"]
OPERATORS = ["-", "*"]

class Number:
    def __init__(self, n): self.n = n
    def __str__(self): return str(self.n)
class Mul:
    def __init__(self, lhs, rhs):
        self.lhs = lhs
        self.rhs = rhs
    def __str__(self): return "(%s*%s)" % (str(self.lhs), str(self.rhs))
class Sub:
    def __init__(self, lhs, rhs):
        self.lhs = lhs
        self.rhs = rhs
    def __str__(self): return "(%s-%s)" % (str(self.lhs), str(self.rhs))

def parse_expr(s, i):
    if s[i] not in NUMBERS:
        return
    j = i
    while j < len(s) and s[j] in NUMBERS:
        j += 1
    lhs = Number(s[i:j])
    if j == len(s) or s[j] not in OPERATORS:
        return (j + 1, lhs)
    op = s[j]
    r = parse_expr(s, j + 1)
    if r is None:
        return
    (i, rhs) = r
    if op == "-":
        return (i, Sub(lhs, rhs))
    else:
        assert op == "*"
        return (i, Mul(lhs, rhs))

def parse(s):
    r = parse_expr(s, 0)
    if r is None or r[0] <= len(s):
        return "Syntax error"
    return r[1]

print(parse("2-3*4"))
print(parse("2*3-4"))
print(parse("2-3-4"))
print(parse("2-3"))
print(parse("2"))
print(parse("2--3"))