Up Down

A Frog present at the bottom of the well of height $H$, needs to get out of it, using the slippery wall of the well. It decides to climb up the well, it goes up $U$ metres and slips down $D$ metres in one single step. So, in each step it covers $(U-D)$ metres, and if the spider gets out of the well by covering $U$ metres in the last step it doesn't slips back. For example, if the spider climbs up