class Unionfind:
    def __init__(self, N):
        self.par = [-1] * N
        self.rank = [1] * N
        
    def root(self, x):
        if self.par[x] < 0:
            return x
        
        self.par[x] = self.root(self.par[x])
        return self.par[x]
    
    def unite(self, x, y):
        rx, ry = self.root(x), self.root(y)
        
        if rx != ry:
            if self.rank[rx] >= self.rank[ry]:
                self.par[rx] += self.par[ry]
                self.par[ry] = rx
                
                if self.rank[rx] == self.rank[ry]:
                    self.rank[rx] += 1
            else:
                self.par[ry] += self.par[rx]
                self.par[rx] = ry
    
    def is_same(self, x, y):
        return self.root(x) == self.root(y)
    
    def count(self, x):
        return -self.par[self.root(x)]

W, H = map(int, input().split())
M = [list(map(int, input().split())) for _ in range(H)]
adj_list = [[] for _ in range(W*H)]
uf = Unionfind(H*W)

for i in range(H):
    for j in range(W):
        for ni, nj in [(i+1, j), (i, j+1)]:
            if 0 <= ni < H and 0 <= nj < W and M[i][j] == M[ni][nj]:
                if uf.is_same(i*W+j, ni*W+nj):
                    print('possible')
                    exit()
                else:
                    uf.unite(i*W+j, ni*W+nj)
    
print('impossible')