title: Strange equality between ceil and floor
body: <p>I was comparing my solution to another's one solution and found this different line which doesn't seem equal at first but they are.</p>
  
  <p>So my question is: How is this equality possible?</p>
  
  <pre><code>ceil(k / x) - 1 == floor((k - 1) / x)
  </code></pre>