import sys

input = sys.stdin.readline


def main():
    W, H = map(int, input().split())
    A = [list(map(int, input().split())) for i in range(H)]

    vis = [[False]*W for i in range(H)]

    def dfs(cx, cy, px, py):
        if vis[cx][cy]:
            return True
        vis[cx][cy] = True
        for dx, dy in [(0, -1), (0, 1), (-1, 0), (1, 0)]:
            nx, ny = cx + dx, cy + dy
            if nx == px and ny == py:
                continue
            if 0 <= nx < H and 0 <= ny < W and A[nx][ny] == A[cx][cy]:
                if dfs(nx, ny, cx, cy):
                    return True

        return False

    for i in range(H):
        for j in range(W):
            if not vis[i][j] and dfs(i, j, -1, -1):
                print('possible')
                return
    print('impossible')


if __name__ == '__main__':
    main()