title: Picking apples off a tree
body: <p>I have the following problem: I am given a tree with N apples, for each apple I am given it's weight and height.  I can pick apples up to a given height H, each time I pick an apple the height of every apple is increased with U.  I have to find out the maximum weight of apples I can pick.   </p>
  
  <p>1 ≤ N ≤ 100000<br>
  0 &lt; {H, U, apples' weight and height, maximum weight} &lt; 2<sup>31</sup>  </p>
  
  <p>Example:</p>
  
  <pre><code>N=4  H=100  U=10  
  
  height weight  
    82     30
    91     10
    93      5
    94     15
  </code></pre>
  
  <p>The answer is 45: first pick the apple with the weight of 15 then the one with the weight of 30.  </p>
  
  <p>Could someone help me approach this problem?</p>