def get_fuel_cost_part2(x1, x2):
    """Get the fuel that would cost move from x1 to x2, when:

    Moving from each change of 1 step in horizontal position costs 1 more unit of fuel than the last. This can be easily
    computed as a triangular number/binomial coefficient.
    """
    steps = abs(x1-x2)
    return steps*(steps+1)/2