#include<bits/stdc++.h>
using namespace std;

#define REP(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)

#define mygc(c) (c)=getchar_unlocked()
#define mypc(c) putchar_unlocked(c)

#define ll long long
#define ull unsigned ll

void reader(int *x){int k,m=0;*x=0;for(;;){mygc(k);if(k=='-'){m=1;break;}if('0'<=k&&k<='9'){*x=k-'0';break;}}for(;;){mygc(k);if(k<'0'||k>'9')break;*x=(*x)*10+k-'0';}if(m)(*x)=-(*x);}
void reader(int *x, int *y){reader(x);reader(y);}
void reader(int *x, int *y, int *z){reader(x);reader(y);reader(z);}
void reader(ll *x){int k,m=0;*x=0;for(;;){mygc(k);if(k=='-'){m=1;break;}if('0'<=k&&k<='9'){*x=k-'0';break;}}for(;;){mygc(k);if(k<'0'||k>'9')break;*x=(*x)*10+k-'0';}if(m)(*x)=-(*x);}
int reader(char c[]){int i,s=0;for(;;){mygc(i);if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF) break;}c[s++]=i;for(;;){mygc(i);if(i==' '||i=='\n'||i=='\r'||i=='\t'||i==EOF) break;c[s++]=i;}c[s]='\0';return s;}

void writer(int x, char c){int s=0,m=0;char f[10];if(x<0)m=1,x=-x;while(x)f[s++]=x%10,x/=10;if(!s)f[s++]=0;if(m)mypc('-');while(s--)mypc(f[s]+'0');mypc(c);}
void writer(ll x, char c){int s=0,m=0;char f[20];if(x<0)m=1,x=-x;while(x)f[s++]=x%10,x/=10;if(!s)f[s++]=0;if(m)mypc('-');while(s--)mypc(f[s]+'0');mypc(c);}
void writer(const char c[]){int i;for(i=0;c[i]!='\0';i++)mypc(c[i]);}

int X, Y;
int mp[111][111];
int vis[111][111];
int fin;

int di[4]={-1,1,0,0},dj[4]={0,0,-1,1};

void dfs(int i, int j, int bi, int bj, int num){
  int d, ni, nj;
  vis[i][j] = 1;
  rep(d,4){
    ni = i+di[d];
    nj = j+dj[d];
    if(ni<0||nj<0||ni>=X||nj>=Y||mp[ni][nj]!=num)continue;
    if(ni==bi&&nj==bj)continue;
    if(vis[ni][nj]){fin=1;continue;}
    dfs(ni,nj,i,j,num);
  }
}

int main(){
  int i, j;

  reader(&Y,&X);
  rep(i,X) rep(j,Y) reader(mp[i]+j);
  rep(i,X) rep(j,Y) if(!vis[i][j]) dfs(i,j,-1,-1,mp[i][j]);
  writer(fin?"possible\n":"impossible\n");

  return 0;
}