Missing Number

Rahul and Neha are playing a simple game. In each turn, they give a list of first N natural numbers to each other in a random order. The challenge is that there is one number missing from the list, and the other person is required to find it. The person who solves the challenge given to him/her first wins that round. Rahul has become good with the game and has been able to win most of them. Neha i