Your car starts at position 0 and speed +1 on an infinite number line. Your car can go into negative positions. Your car drives automatically according to a sequence of instructions 'A' (accelerate) and 'R' (reverse):

When you get an instruction 'A', your car does the following:

• position += speed
• speed *= 2

When you get an instruction 'R', your car does the following:

• speed = 1
• Reversing its direction
• The position stays the same.

For example, after commands "AAR", your car goes to positions 0 --> 1 --> 3 --> 3, and your speed goes to 1 --> 2 --> 4 --> -1.

Given a sequence of instructions, return the distance that a car has moved.