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 not inner_range(ny, nx):
            continue
        
        if area[y][x] != area[ny][nx]:
            continue
        
        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 + 2)] for _ in range(H + 2)]

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("possiple")
            exit(0)
        
print("impossible")