from collections import deque

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

visited = []
for _ in range(H):
    visited.append([False for _ in range(W)])

dh = [ 0, -1, 0, 1]
dw = [-1,  0, 1, 0]
    
def search(y,x):
    number = M[y][x]
    que = deque()
    que.appendleft((y,x))
    while len(que) != 0:
        h,w = que.popleft()
        visited[h][w] = True
        for i in range(len(dh)):
            nh = h + dh[i]
            nw = w + dw[i]
            if 0 <= nh < H and 0 <= nw < W and \
               visited[nh][nw] == False and M[nh][nw] == number:
                que.appendleft((nh,nw))
    return found()

def found():
    for h in range(H):
        for w in range(W):
            if visited[h][w]:
                count = 0
                for i in range(len(dh)):
                    nh = h + dh[i]
                    nw = w + dw[i]
                    if 0 <= nh < H and 0 <= nw < W and visited[nh][nw]:
                        count = count + 1
                if count < 2:
                    return False
    return True

def reset():
    for h in range(H):
        for w in range(W):
            visited[h][w] = False

possible = False
for i in range(H):
    for j in range(W):
        if search(i,j):
            possible = True
            break
        reset()
    if possible:
        break

if possible:
    print('possible')
else:
    print('impossible')