if ((n == 1) || (n == 0))

#include <iostream>
#include <cmath>

unsigned __int64 fact(unsigned __int64 n)
{
	if (n < 2)
	{
		return 1;
	}
	else
	{
		return n * fact(n - 1);
	}
}

int main()
{
	unsigned __int64 n;
		do
		{
			std::cout << "Enter n (n >= 1 and n = 1 (mod 2)):\r\n";
			std::cin >> n;
		} while ((n < 1) || (n % 2 == 0));
	std::cout << "Enter x:\r\n";
	double x;
	std::cin >> x;
	double summ = 0.0;
		for (unsigned __int64 i = 1; i <= n; i += 2)
		{
			summ += pow(x, (double)i) / fact(i);
		}
	std::cout << "Result: " << summ << "\r\n\r\n";
	return 0;
}