I cant solve expression involving str.find()
<p>I got a problem:</p>

<blockquote>
  <p>Variables <code>s1</code> and <code>s2</code> refer to strs. The expression <code>s1.find(s2)</code>
  returns the index of the first occurrence of s2 in s1.</p>
  
  <p>Write an expression that gives the index of the second occurrence of
  <code>s2</code> in <code>s1</code>. If <code>s2</code> does not occur twice in <code>s1</code>, the expression
  should evaluate to -1. Unlike <code>str.count</code>, you should allow
  overlapping occurrences of <code>s2</code>.</p>
  
  <p>Your answer must be a single expression that does not use the square
  bracket notation, and you can only call method str.find and use the
  arithmetic operators (+, -, etc.).</p>
</blockquote>

<p>Is this answer correct?</p>

<pre><code>s1.find('s2',(s1.find('s2')+1))
</code></pre>

<p>Thanks</p>
