Plumbing the Pond

Bessie drinks water from a pond in the northwest part of the farm. The pond has an interesting bottom: it is full of little hills and valleys, and she wonders how deep it is.

She trolls across the pond in her little boat using a very old radar set that tends to produce spurious readings. Because the deepest part of the pond is relatively flat, she decides to trust a large depth value only if the same value also appears in an adjacent reading.

The pond is modeled as an $R \times C$ grid ($1 \le R \le 50$, $1 \le C \le 50$) of depth readings $D_{r,c}$ ($0 \le D_{r,c} \le 1{,}000{,}000$). A reading of $0$ marks a cell that is not part of the pond; a reading of $10$ means "depth of $10$".

Two cells are adjacent if one lies in any of the (up to eight) cells that border the other horizontally, vertically, or diagonally. Find the greatest depth value that appears in at least two adjacent cells. It is guaranteed that at least one pair of equal, positive, adjacent readings exists.

• Line 1: Two space-separated integers $R$ and $C$.
• Lines 2 to $R+1$: line $i+1$ contains the $C$ space-separated depth readings of row $i$: $D_{i,1}, D_{i,2}, \dots, D_{i,C}$.

• Line 1: A single integer, the depth of the pond determined by Bessie's rule.

The deepest single reading need not be the answer. Only depths that occur in two adjacent cells are candidates, and the answer is the largest such candidate. A value that appears several times but never in two adjacent cells does not qualify.