#!/usr/bin/env python3

import array
import collections
import itertools


REC_LIMIT = 10000
DELTAS = [(1, 0), (-1, 0), (0, 1), (0, -1)]
UNDEF = (-1, -1)


class Solver(object):

    def __init__(self, width, height, field):
        self.width = width
        self.height = height
        self.field = field
        self.visited = collections.defaultdict(bool)
        self.pred = collections.defaultdict(lambda: UNDEF)

    def in_field(self, r, c):
        return 0 <= r < self.height and 0 <= c < self.width

    def in_cycle(self, r0, c0, num):
        self.visited[(r0, c0)] = True
        q = collections.deque()
        q.append((r0, c0))
        while q:
            (r1, c1) = q.pop()
            for dr, dc in DELTAS:
                (r, c) = (r1 + dr, c1 + dc)
                if not self.in_field(r, c):
                    continue
                elif self.field[r][c] != num:
                    continue
                if self.visited[(r, c)]:
                    if self.pred[(r1, c1)] != (r, c):
                        return True
                    else:
                        continue
                self.visited[(r, c)] = True
                self.pred[(r, c)] = (r1, c1)
                q.append((r, c))
        return False

    def judge(self):
        rcs = itertools.product(range(self.height), range(self.width))
        for r, c in rcs:
            if not self.visited[(r, c)]:
                answer = self.in_cycle(r, c, self.field[r][c])
                if answer:
                    return True
        else:
            return False


def main():
    width, height = map(int, input().split())
    field = [array.array("I", map(int, input().split()))
             for _ in range(height)]
    solver = Solver(width, height, field)
    print("possible" if solver.judge() else "impossible")


if __name__ == '__main__':
    main()