import sys
input = sys.stdin.readline

class UnionFind(object):
    def __init__(self, n=1):
        self.par = [i for i in range(n)]
        self.rank = [0 for _ in range(n)]
        self.size = [1 for _ in range(n)]

    def find(self, x):
        if self.par[x] == x:
            return x
        else:
            self.par[x] = self.find(self.par[x])
            return self.par[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)
        if x != y:
            if self.rank[x] < self.rank[y]:
                x, y = y, x
            if self.rank[x] == self.rank[y]:
                self.rank[x] += 1
            self.par[y] = x
            self.size[x] += self.size[y]

    def is_same(self, x, y):
        return self.find(x) == self.find(y)

    def get_size(self, x):
        x = self.find(x)
        return self.size[x]
    

W, H = map(int, input().split())
M = []
for i in range(H):
    M.append(list(map(int, input().split())))

for n in range(1005):
    U = UnionFind(10**4+5)
    for a in range(H):
        for b in range(W):
            if a != H - 1:
                if M[a][b] == M[a + 1][b] == n:
                    i1 = a * W + b
                    i2 = (a + 1) * W + b
                    if U.is_same(i1, i2):
                        print("possible")
                        exit()
                    U.union(i1, i2)
            if b != W - 1:
                if M[a][b] == M[a][b + 1] == n:
                    i1 = a * W + b
                    i2 = a * W + b + 1
                    if U.is_same(i1, i2):
                        print("possible")
                        exit()
                    U.union(i1, i2)

print("impossible")