from collections import deque

def bfs_matrix(grid):
    rows, cols = len(grid), len(grid[0])
    queue = deque()
    visited = set()
    
    # 1. Initialize (Add all starting points)
    # Example: Find all '2's (rotten oranges)
    for r in range(rows):
        for c in range(cols):
            if grid[r][c] == 2:
                queue.append((r, c))
                visited.add((r, c))
    
    steps = 0
    directions = [(0, 1), (0, -1), (1, 0), (-1, 0)] # Right, Left, Down, Up
    
    # 2. The Loop
    while queue:
        # Range(len) is crucial to distinguish "layers" (distance)
        for _ in range(len(queue)):
            r, c = queue.popleft()
            
            # 3. Check neighbors
            for dr, dc in directions:
                nr, nc = r + dr, c + dc
                
                # 4. Validate Bounds & Visited
                if (0 <= nr < rows and 0 <= nc < cols and 
                    (nr, nc) not in visited and 
                    grid[nr][nc] == 1): # Example condition: Fresh orange
                    
                    visited.add((nr, nc))
                    queue.append((nr, nc))
        steps += 1
        
    return steps