Help me in solving LB08A problem

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Learning course: Python for problem solving - 2
Problem Link: CodeChef: Practical coding for everyone

The problem states that u have N friends and M left shoes. U have to gift each one of your friend a pair of shoes . So how many minimum numbers of shoes u need to buy to make it possible.
if N<=M
u need to buy N more shoes which is the right pair .
u need to buy M right pair shoes and (N-M)both pair shoes .