def fibonacci_iterative(n):
    if n < 0:
        raise ValueError("Input must be a non-negative integer.")
    
    # Base cases for the first two Fibonacci numbers
    fib_1 = 0
    fib_2 = 1
    
    # Handle small inputs directly to avoid unnecessary calculations
    if n in [0, 1]:
        return n

    else:
        i = 3     # starting index of a new sequence (step)
        while i <= n:   
            # Sum the last two numbers and set it as current number
            fib_next = fib_1 + fib_2
            
            # Move forward by one step in Fibonacci's sequence
            i += 1

            # Update "old" values to reflect this new value
            fib_1, fib_2 = fib_2, fib_next
        
        return fib_2

# Test the function with a non-trivial example
print(fibonacci_iterative(9))