Add up all the climbing you do uphill and disregard the downhills.
So, if you were to start at sea level, go consistently uphill to an elevation +1000 feet, the consistently downhill to an elevation of 500 feet, then consistently uphill back to the elevation of +1000 feet, then finally, a consistent downhill back to sea level, your total climb would be 1500 feet.
__________________
Proud parent of a happy inner child ...