アルゴリズム86-breakingチョコレート問題

Q.
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.
A)

function breakChocolate(n,m) {
  if (n <= 0 || m <= 0)
    return 0;
  else
    return (n * m) - 1;
}