def POS(i, j, w):
    return i * (w + 2) + j
def dfs(x, y, dir, number, board, w, h):
    dx = [1, 0, -1, 0]
    dy = [0, 1, 0, -1]
    board[POS(x, y, w)] = -number
    for k in range(4):
        if dir == k:
            continue
        nx = x + dx[k]
        ny = y + dy[k]
        if board[POS(nx, ny, w)] == -number:
            return 1
        if board[POS(nx, ny, w)] == number:
            t = dfs(nx, ny, (k + 2) % 4, number, board, w, h)
            if t == 1:
                return 1
    return 0
def main():
    w, h = map(int,input().split())
    board = [0] * ((w + 2) * (h + 2))
    for i in range(1, h + 1):
        row = list(map(int, input().split()))
        for j in range(1, w + 1):
            board[POS(i, j, w)] = row[j - 1]
    for i in range(1,h + 1):
        for j in range(1,w + 1):
            if board[POS(i, j, w)] > 0:
                res = dfs(i, j, 4, board[POS(i, j, w)], board, w, h)
                if res == 1:
                    print("possible")
                    exit(0)
    print("impossible")
    return
if __name__ == "__main__":
    main()