import sys
input = sys.stdin.readline

W, H = map(int, input().split())
M = [list(map(int, input().split())) for _ in range(H)]
used = [[False] * W for _ in range(H)]
for si in range(H):
    for sj in range(W):
        if used[si][sj]:
            continue
        st = [(si, sj, -1, -1)]
        used[si][sj] = True
        while st:
            i, j, pi, pj = st.pop()
            for di, dj in [(1, 0), (0, 1), (-1, 0), (0, -1)]:
                ni, nj = i + di, j + dj
                if 0 <= ni < H and 0 <= nj < W and M[ni][nj] == M[i][j] and (ni, nj) != (pi, pj):
                    if used[ni][nj]:
                        print('possible')
                        exit()
                    st.append((ni, nj, i, j))
                    used[ni][nj] = True
print('impossible')