int FibByPos(int pos)
{
    if (pos <= 0) return -1;
    if (pos == 1 || pos == 2) return 1;
    
    int fib1 = 1, fib2 = 1, fib_current;
    for (int i = 3; i <= pos; i++)
    {
        fib_current = fib1 + fib2;
        fib1 = fib2;
        fib2 = fib_current;
    }
    return fib_current;
}