Get Longest Substring in Python

def main():
	print()
	exp = input("Enter a math expression: ")

	stack = []
	for c in exp:
		if c == "(":
			stack.append(c)
		elif c == ")":
			if stack:
				stack.pop()
			else:
				# More closing than opening → invalid
				print("The math expression is invalid.")
				return

	if not stack:
		print("The math expression is valid.")
	else:
		print("The math expression is invalid.")


if __name__ == "__main__":
	main()