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)]

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
            if y!=0:
                if M[y-1][x]==M[y][x]:
                    if visit[y-1][x]:
                        cnt+=1
                    else:
                        visit[y-1][x]=1
                        Q.append((y-1,x))
            if y!=h-1:
                if M[y+1][x]==M[y][x]:
                    if visit[y+1][x]:
                        cnt+=1
                    else:
                        visit[y+1][x]=1
                        Q.append((y+1,x))
            if x!=0:
                if M[y][x-1]==M[y][x]:
                    if visit[y][x-1]:
                        cnt+=1
                    else:
                        visit[y][x-1]=1
                        Q.append((y,x-1))
            if x!=w-1:
                if M[y][x+1]==M[y][x]:
                    if visit[y][x+1]:
                        cnt+=1
                    else:
                        visit[y][x+1]=1
                        Q.append((y,x+1))
            if cnt>=2:
                print("possible")
                exit()
    
print("impossible")