Breaking Chocolate Problem

Your task is to split the chocolate bar of given dimension n x m into small squares. Each square is of size 1x1 and unbreakable. Implement a function that will return minimum number of breaks needed.

For example if you are given a chocolate bar of size 2 x 1 you can split it to single squares in just one break, but for size 3 x 1 you must do two breaks.

If input data is invalid you should return 0 (as in no breaks are needed if we do not have any chocolate to split). Input will always be a non-negative integer.

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def break_chocolate(n, m):
    if n==0 or m==0:
        return 0
    return n*m-1