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

n=W*H+3

# UnionFind

Group = [i for i in range(n+1)] # グループ分け
Nodes = [1]*(n+1) # 各グループのノードの数

def find(x):
    while Group[x] != x:
        x=Group[x]
    return x

def Union(x,y):
    if find(x) != find(y):
        if Nodes[find(x)] < Nodes[find(y)]:
            
            Nodes[find(y)] += Nodes[find(x)]
            Nodes[find(x)] = 0
            Group[find(x)] = find(y)
            
        else:
            Nodes[find(x)] += Nodes[find(y)]
            Nodes[find(y)] = 0
            Group[find(y)] = find(x)

COUNT=[[0]*W for i in range(H)]


for i in range(H):
    for j in range(W):
        a=M[i][j]
        count=0

        for x,y in [(i+1,j),(i-1,j),(i,j+1),(i,j-1)]:
            if 0<=x<H and 0<=y<W:
                if M[x][y]==a:
                    count+=1
        COUNT[i][j]=count

Q=[]
for i in range(H):
    for j in range(W):
        if COUNT[i][j]<=1:
            Q.append((i,j))

while Q:
    i,j=Q.pop()
    a=M[i][j]
    if a==-1:
        continue

    for x,y in [(i+1,j),(i-1,j),(i,j+1),(i,j-1)]:
        if 0<=x<H and 0<=y<W:
            if M[x][y]==a:
                COUNT[x][y]-=1
                if COUNT[x][y]<=1:
                    Q.append((x,y))
    M[i][j]=-1
    
    

for i in range(H):
    for j in range(W):
        a=M[i][j]
        if a==-1:
            continue
        count=0

        for x,y in [(i+1,j),(i-1,j),(i,j+1),(i,j-1)]:
            if 0<=x<H and 0<=y<W:
                if M[x][y]==a:
                    Union(x*W+y,i*W+j)

if max(Nodes)>=4:
    print("possible")
else:
    print("impossible")