w, h = map(int, input().split())
M = [list(map(int, input().split())) for _ in range(h)]

Directions = [(1, 0), (0, 1), (-1, 0), (0, -1)]
visited = [[False for _ in range(w)] for _ in range(h)]
for i in range(h):
    for j in range(w):
        if visited[i][j]:
            continue
        num = M[i][j]
        visited[i][j] = True
        Stack = [(i, j, -1, -1)]
        while Stack:
            ci, cj, pi, pj = Stack.pop()
            for di, dj in Directions:
                ni = ci + di
                nj = cj + dj
                if 0 <= ni < h and 0 <= nj < w:
                    if M[ni][nj] != num:
                        continue
                    if ni == pi and nj == pj:
                        continue
                    if visited[ni][nj]:
                        print("possible")
                        exit()
                    visited[ni][nj] = True
                    Stack.append((ni, nj, ci, cj))
print("impossible")