class UnionFind:
    def __init__(self, n):
        self.n=n
        self.par=[-1]*n
    def find(self, x):
        if self.par[x]<0:
            return x
        self.par[x]=self.find(self.par[x])
        return self.par[x]
    def unite(self, x, y):
        x=self.find(x)
        y=self.find(y)
        if x==y:
            return False
        if self.par[x]<self.par[y]:
            x, y=y, x
        self.par[y]+=self.par[x]
        self.par[x]=y
        return True
    def same(self, x, y):
        return self.find(x)==self.find(y)
    def size(self, x):
        return -self.par[self.find(x)]
w, h=map(int, input().split())
m=[]
for i in range(h):
    m.append(list(map(int, input().split())))
uf=UnionFind(h*w)
for i in range(h):
    for j in range(w-1):
        if m[i][j]==m[i][j+1]:
            if not uf.unite(i*w+j, i*w+j+1):
                print("possible")
                exit()
for i in range(h-1):
    for j in range(w):
        if m[i][j]==m[i+1][j]:
            if not uf.unite(i*w+j, (i+1)*w+j):
                print("possible")
                exit()
print("impossible")