If you have not ever heard the term **Arithmetic Progrossion**, refer to: 
http://www.codewars.com/kata/find-the-missing-term-in-an-arithmetic-progression/python
And here is an unordered version. Try if you can survive lists of **MASSIVE** numbers (which means time limit should be considered). :D
Note: Don't be afraid that the minimum or the maximum element in the list is missing, e.g. [4, 6, 3, 5, 2] is missing 1 or 7, but this case is excluded from the kata.
Example:
```python
find([3, 9, 1, 11, 13, 5]) # => 7
```
def find(seq):
  seq.sort()
  pattern = abs(seq[0]-seq[1])
  for i in range(len(seq)-1):
    if seq[i]!=seq[i+1]-pattern:
      return seq[i]+pattern
  

def find(seq):
  seq.sort()
  pattern = abs(seq[0]-seq[1])
  for i in range(len(seq)-1):
    if seq[i]!=seq[i+1]-pattern:
      return seq[i]+pattern
  
