# Powerful Squares

Let $$(m, n)$$ be an ordered pair of positive integers such that $$5^m - 7^n$$ is a perfect square. Find the maximum possible value of $$m + n$$.

If you think there is no maximum value, i.e. there are infinitely many pairs $$(m, n)$$ that satisfy the condition, input $$-1$$ as your answer.

Note: A perfect square must be nonnegative.

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