class Unionfind:
     
    def __init__(self,n):
        self.uf = [-1]*n
 
    def find(self,x):
        if self.uf[x] < 0:
            return x
        else:
            self.uf[x] = self.find(self.uf[x])
            return self.uf[x]
 
    def same(self,x,y):
        return self.find(x) == self.find(y)
 
    def union(self,x,y):
        x = self.find(x)
        y = self.find(y)
        if x == y:
            return False
        if self.uf[x] > self.uf[y]:
            x,y = y,x
        self.uf[x] += self.uf[y]
        self.uf[y] = x
        return True
 
    def size(self,x):
        x = self.find(x)
        return -self.uf[x]


w,h = map(int,input().split())
M = [list(map(int,input().split())) for i in range(h)]
for i in range(h-1):
    for j in range(w-1):
        if M[i][j] == M[i][j+1] == M[i+1][j] == M[i+1][j+1]:
            print("possible")
            exit()

uf = Unionfind(w*h)
for i in range(h):
    for j in range(w):
        if i < h-1 and M[i][j] == M[i+1][j]:
            x = i*w+j
            y = (i+1)*w+j
            if uf.same(x,y):
                print("possible")
                exit()
            uf.union(x,y)

        if j < w-1 and M[i][j] == M[i][j+1]:
            x = i*w+j
            y = (i)*w+j+1
            if uf.same(x,y):
                print("possible")
                exit()
            uf.union(x,y)

print("impossible")