Given an m by n matrix mat where each row is sorted in strictly increasing order, return the smallest element that appears in every row of mat. If no such common element exists, return $-1$.

Constraints:

• m == mat.length

• n == mat[i].length

• $1 \leq$ m, n $\leq 500$

• $1 \leq$ mat[i][j] $\leq 10^4$

• mat[i] is sorted in strictly increasing order.

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