W, H = map(int, input().split())

M = [list(map(int, input().split())) for i in range(H)]


visited = [[0] * W + [-1] for i in range(H)] + [[-1] * (W + 1)]

from collections import deque

for i in range(H):
    
    for j in range(W):
        
        if visited[i][j] == 0:
            que = []
            X = M[i][j]
            if visited[i + 1][j] == 0 and M[i + 1][j] == X:
                visited[i + 1][j] = 1
                que += [i + 1, j, 0]
                
            if visited[i][j + 1] == 0 and M[i][j + 1] == X:
                visited[i][j + 1] = 1
                que += [i, j + 1, 1]
                
            if visited[i - 1][j] == 0 and M[i - 1][j] == X:
                visited[i - 1][j] = 1
                que += [i - 1, j, 2]
                
            if visited[i][j - 1] == 0 and M[i][j - 1] == X:
                visited[i][j - 1] = 1
                que += [i][j - 1, 3]
            visited[i][j] = 1
            
            que = deque(que)
            
            while que:
                x, y, q = que.popleft(), que.popleft(), que.popleft()
                
                if visited[x - 1][y] != -1 and M[x - 1][y] == X and q != 0:
                    if visited[x - 1][y]:
                        print('possible')
                        exit()
                        
                    else:
                        
                        visited[x - 1][y] = 1
                        que.extend([x - 1, y, 2])
                if visited[x + 1][y] != -1 and M[x + 1][y] == X and q != 2:
                    
                    if visited[x + 1][y]:
                        print('possible')
                        exit()
                        
                    else:
                        visited[x + 1][y] = 1
                        que.extend([x + 1, y, 0])
                    
                if visited[x][y - 1] != -1 and M[x][y - 1] == X and q != 1:
                    
                    if visited[x][y - 1]:
                        print('possible')
                        exit()
                    else:
                        visited[x][y - 1] = 1
                        que.extend([x, y - 1, 3])
                
                if visited[x][y + 1] != -1 and M[x][y + 1] == X and q != 3:
                    if visited[x][y + 1]:
                        print('possible')
                        exit()
                        
                    else:
                        
                        visited[x][y + 1] = 1
                        que.extend([x, y + 1, 1])
                    
print('impossible')