bool IsFib(int n)
{
    if (n <= 0) return false;
    
    int fib1 = 1, fib2 = 1;
    
    if (n == 1) return true;
    
    while (fib2 < n)
    {
        int temp = fib2;
        fib2 = fib1 + fib2;
        fib1 = temp;
    }
    
    return (fib2 == n);
}