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

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

memo = [[0] * W for _ in range(H)]
import sys
for h in range(H):
    for w in range(W):
        if memo[h][w] == 1:continue
        stack = [(h,w,-1,-1)]
        memo[h][w] = 1
        while stack:
            h,w,bh,bw = stack.pop()
            for i,j in [(1,0),(-1,0),(0,1),(0,-1)]:
                if 0 <= h + i < H and 0 <= w + j < W:
                    if h + i != bh or w + j != bw:
                        if S[h+i][w+j] == S[h][w]:
                            if memo[h+i][w+j] == 1:
                                print("possible")
                                exit()
                            else:
                                stack.append((h+i,w+j,h,w))
                                memo[h+i][w+j] = 1
print("impossible")