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):
    found = False
    number = M[y][x]
    que = deque()
    que.appendleft( ((-1,-1),(y,x)) )
    while len(que) != 0:
        back,now = que.popleft()
        u,v = back
        h,w = now
        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 \
               (nh != u or nw != v) and M[nh][nw] == number:
                if visited[nh][nw]:
                    found = True
                    break
                else:
                    que.appendleft( ((h,w), (nh,nw)) )
    return found

def reset():
    for i in range(H):
        for j in range(W):
            visited[i][j] = 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')