Let $x$ and $y$ be real numbers such that $0 \le x \le 1$ and $0 \le y \le 1.$  Find the maximum value of
\[x^2 y - xy^2.\]The answer is in the form rac{m}{n}, where gcd(m, n) = 1. Please provide the value of m + n.

Let's think step by step and output the final answer within \boxed{}.
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