title: Find the number of pairs in list
body: <p>Suppose <code>lst = [7,1,5,4,2,3,6]</code>, <code>(7, 2), (5, 4), (6, 3)</code> are some of the pairs and in total there are 6 pairs that adds up to  <code>9</code></p>
  
  <p>(i) The order of numbers in a pair matters. For example, (7, 2) and (2, 7) are two different pairs.
  (ii) A number cannot pair with itself.
  (iii) There is no duplicate element in the list</p>
  
  <pre><code>def find_pairs(lst, key):
      count = 0
      if sum(lst[count:count+1]) == key:
          count += 1
          return count
      else:
          return find_pairs(lst[1:],key)  
  </code></pre>
  
  <p>This is my code. What's wrong ?? I am getting an error
  input <code>find_pairs([7,1,5,4,2,3,6], 9)</code> give <code>6</code></p>
  
  <pre><code>find_pairs(list(range(1, 100, 2)), 55) #0
  find_pairs(list(range(1, 100, 2)), 56) #28
  </code></pre>