#!/usr/bin/env python3
import sys
input = sys.stdin.readline

w, h = map(int, input().split())
field = []
visited = [[0] * (w+2) for _ in range(h+2)]
field.append([-1] * (w + 2))
for _ in range(h):
    line = [-1] + [int(item) for item in input().split()] + [-1]
    field.append(line)
field.append([-1] * (w + 2))


delta = [(1, 0), (0, 1), (-1, 0), (0, -1)]
def dfs(sx, sy):
    st = [(sx, sy, -1, -1)]
    c = field[sx][sy]
    visited[sx][sy] = 1
    while st:
        x, y, px, py = st.pop()
        for dx, dy in delta:
            if x + dx == px and y + dy == py:
                continue
            if field[x + dx][y + dy] == c:
                if not visited[x + dx][y + dy]:
                    visited[x + dx][y + dy] = 1
                    st.append((x+dx, y+dy, x, y))
                elif visited[x + dx][y + dy]:
                    print("possible")
                    exit()
for i in range(1, 1+h):
    for j in range(1, 1+w):
        if visited[i][j]:
            continue
        dfs(i, j)
print("impossible")