1207B · Square Filling

Problem: You are given two matrices $A$ and $B$. Each matrix contains exactly $n$ rows and $m$ columns. Each element of $A$ is either $0$ or $1$; each element of $B$ is initially $0$.

You may perform some operations with matrix $B$. During each operation, you choose any submatrix of $B$ having size $2 \times 2$, and replace every element in the chosen submatrix with $1$. In other words, you choose two integers $x$ and $y$ such that $1 \le x < n$ and $1 \le y < m$, and then set $B_{x, y}$, $B_{x, y + 1}$, $B_{x + 1, y}$ and $B_{x + 1, y + 1}$ to $1$.

Your goal is to make matrix $B$ equal to matrix $A$. Two matrices $A$ and $B$ are equal if and only if every element of matrix $A$ is equal to the corresponding element of matrix $B$.

Is it possible to make these matrices equal? If it is, you have to come up with a sequence of operations that makes $B$ equal to $A$. Note that you don't have to minimize the number of operations.

Input Format: The first line contains two integers $n$ and $m$ ($2 \le n, m \le 50$).

Then $n$ lines follow, each containing $m$ integers. The $j$-th integer in the $i$-th line is $A_{i, j}$. Each integer is either $0$ or $1$.

Output Format: If it is impossible to make $B$ equal to $A$, print one integer $-1$.

Otherwise, print any sequence of operations that transforms $B$ into $A$ in the following format: the first line should contain one integer $k$ — the number of operations, and then $k$ lines should follow, each line containing two integers $x$ and $y$ for the corresponding operation (set $B_{x, y}$, $B_{x, y + 1}$, $B_{x + 1, y}$ and $B_{x + 1, y + 1}$ to $1$). The condition $0 \le k \le 2500$ should hold.

Note: The sequence of operations in the first example:

$\begin{matrix} 0 & 0 & 0 & & 1 & 1 & 0 & & 1 & 1 & 1 & & 1 & 1 & 1 \\ 0 & 0 & 0 & \rightarrow & 1 & 1 & 0 & \rightarrow & 1 & 1 & 1 & \rightarrow & 1 & 1 & 1 \\ 0 & 0 & 0 & & 0 & 0 & 0 & & 0 & 0 & 0 & & 0 & 1 & 1 \end{matrix}$