import sys

def ok(y, x):
    return y >= 0 and x >= 0 and y < H and x < W

dx = [1, 0, -1, 0]
dy = [0, 1, 0, -1]

def dfs(y, x, pre):
    check[y][x] = True
    for k in range(4):
        if pre == k:
            continue
        ny = y + dy[k]
        nx = x + dx[k]
        if not ok(ny, nx):
            continue
        if field[y][x] != field[ny][nx]:
            continue
        if check[ny][nx]:
            return True
        if dfs(ny, nx, (k + 2) % 4):
            return True

    return False

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

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

check = [[False for _ in range(W)] for _ in range(H)]

for i in range(H):
    for j in range(W):
        if check[i][j]:
            continue
        if dfs(i, j, -1):
            print("possible")
            sys.exit(0)

print("impossible")