from collections import deque

w,h=map(int,input().split())

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

visit=[[0 for _ in range(w)] for _ in range(h)]

vx=[0,1,0,-1]
vy=[1,0,-1,0]

for i in range(h):
    for j in range(w):
        if visit[i][j]:
            continue
        Q=deque()
        visit[i][j]=1
        Q.append((i,j))
        while(Q):
            y,x=Q.pop()
            cnt=0
            for k in range(4):
                ny=y+vy[k]
                nx=x+vx[k]
                if 0<=ny<len(M) and 0<=nx<len(M[0]):
                    if M[ny][nx]==M[y][x]:
                        if visit[ny][nx]:
                            cnt+=1
                        else:
                            visit[ny][nx]=1
                            Q.append((ny,nx))
            if cnt>=2:
                print("possible")
                exit()
    
print("impossible")