Factorial Symmetry

$\large a! b! = a! + b!$

Let all the pairs of positive integer solutions of $$(a,b)$$ satisfying the equation above be $$(a_1, b_1) , (a_2, b_2) , \ldots , (a_n , b_n)$$. Find $(a_1 + b_1) + (a_2 + b_2) + \cdots + (a_n + b_n) .$

Notation: $$!$$ denotes the factorial notation. For example, $$8! = 1\times2\times3\times\cdots\times8$$.

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