I wish

There is absolutely no need to add a check for each individual number, just do this:

#include 
#include 


int main()
{
	int number = 0;
	int numberToAdd = 1;
	int modifier = 1;

	std::cout << "Is your number [p]ositive or [n]egative? (Default: positive)\n";
	if (std::cin.get() == 'n') {
		modifier *= -1;
	}

	std::cin.ignore(std::numeric_limits::max(), '\n'); // Clear the input buffer

	bool isEven = true;
	bool running = true;

	while (running) {
		std::cout << number << " is " << (isEven ? "even" : "odd") << ".\n";
		std::cout << "Continue? [y/n] (Default: yes)\n";

		if (std::cin.peek() == 'n') {
			running = false;
		}

		number += numberToAdd * modifier;
		isEven = !isEven;

		std::cin.ignore(std::numeric_limits::max(), '\n');
	}

	std::cout << "Your number, " << number << " was " << (isEven ? "even" : "odd") << ".\n";
}```