import sys

def inner_range(y, x):
    return y >= 0 and x >= 0 and y < H and x < W

def check(y, x, py, px):
    matrix[y][x] = True
    
    for dy, dx in zip(DY, DX):
        ny = y + dy
        nx = x + dx
        
        if nx == px and ny == py:
            continue
        
        if 0 <= ny < H and 0 <= nx < W and area[y][x] == area[ny][nx]:
            if matrix[ny][nx]:
                return True
        
            if check(ny, nx, y, x):
                return True
        
    return False
        

W, H = map(int, input().split())
matrix = [[False for _ in range(W)] for _ in range(H)]

DY = (1, 0, -1, 0)
DX = (0, 1, 0, -1)

area = []
for _ in range(H):
    area.append(list(map(int, input().split())))

judge = False
for i in range(H):
    for j in range(W):
        if matrix[i][j]:
            continue
        
        if check(i, j, -1, -1):
            print("possible")
            sys.exit()
        
print("impossible")