title: Please help me to understand this codility test
body: <p>Given the codility test below:</p>
  
  <blockquote>
    <p>Given an array <code>A</code> of <code>N</code> integers, we draw <code>N</code> discs in a 2D plane such that the <code>I</code>-th
    disc is centered on <code>(0,I)</code> and has a radius of <code>A[I]</code>. We say that the <code>J</code>-th disc and
    K<code>-th disc intersect if</code>J ≠ K<code>and</code>J<code>-th and</code>K`-th discs have at least one common
    point.  </p>
    
    <p>Write a function <code>class Solution { public int solution(int[] A); }</code> that, given an array
    <code>A</code> describing <code>N</code> discs as explained above, returns the number of pairs of intersecting
    discs. For example, given <code>N=6</code> and:  </p>
    
    <p><code>A[0] = 1  A[1] = 5  A[2] = 2</code><br>
       <code>A[3] = 1  A[4] = 4  A[5] = 0</code>    </p>
    
    <p>Intersecting discs appear in <strong>eleven pairs of elements</strong>:  </p>
    
    <p>0 and 1,<br>
       0 and 2,<br>
       0 and 4,<br>
       1 and 2,<br>
       1 and 3,<br>
       1 and 4,<br>
       1 and 5,<br>
       2 and 3,<br>
       2 and 4,<br>
       3 and 4,<br>
       4 and 5.</p>
    
    <p>so the function should return <code>11</code>.  </p>
    
    <p>The function should return <code>−1</code> if the number of intersecting pairs exceeds 10,000,000.<br>
    Assume that: </p>
    
    <p>-<code>N</code>is an integer within the range <code>[0..100,000]</code>;<br>
    - Each element of array A is an integer within the range [0..2147483647].  </p>
    
    <p><strong>Complexity</strong>  </p>
    
    <ul>
    <li>Expected worst-case time complexity is O(N*log(N));  </li>
    <li>Expected worst-case space complexity is O(N), beyond input storage (not counting the
    storage required for input arguments).  </li>
    </ul>
  </blockquote>
  
  <p>Where do these eleven pairs come from, since there are only 6 elements?</p>