# A child is playing with a ball on the nth floor of a tall building. The height of this floor, h, is known.
#
# He drops the ball out of the window. The ball bounces (for example), to two-thirds of its height (a bounce of 0.66).
#
# His mother looks out of a window 1.5 meters from the ground.
#
# How many times will the mother see the ball pass in front of her window (including when it's falling and bouncing?
#
# Three conditions must be met for a valid experiment:
# Float parameter "h" in meters must be greater than 0
# Float parameter "bounce" must be greater than 0 and less than 1
# Float parameter "window" must be less than h.
# If all three conditions above are fulfilled, return a positive integer, otherwise return -1.


def bouncing_ball(h, bounce, window):
    result = 0
    if not h > 0 or not 0 < bounce < 1 or not window < h:
        return -1
    else:
        while h > window:
            h *= bounce
            result += 2
        return result - 1


height = float(input())
bounce = float(input())
window_height = float(input())
print(bouncing_ball(height, bounce, window_height))
