Given an m $\times$ n integer matrix mat and an integer k, return a matrix answer of size m $\times$ n where each cell answer[i][j] is the sum of all elements mat[r][c] that satisfy:

• $i - k \leq r \leq i + k$

• $j - k \leq c \leq j + k$

• $0 \leq r <$ m

• $0 \leq c <$ n

Constraints:

• m == mat.length

• n == mat[i].length

• $1 \leq$ m, n, k $\leq 100$

• $1 \leq$ mat[i][j] $\leq 100$

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