Pyton using data structure : How to write a logic for the following 
 Every 3rd element of the input queue  from a front-> rear should be added to output queue
Otherwise it should return -1
Example:l=[1,2,3,4,5,6,7,8,9,10]
It should return 4,7,10 
Exa:l=[1,2]
It should return -1