#include <bits/stdc++.h>
using namespace std;
int main() {
    int n, m; cin >> n >> m;
    vector<vector<int>> g(n, vector<int>(m));
    queue<pair<int,int>> q;
    int fresh = 0;
    for (int i=0;i<n;i++) for (int j=0;j<m;j++) {
        cin >> g[i][j];
        if (g[i][j]==2) q.push({i,j});
        else if (g[i][j]==1) fresh++;
    }
    int mins=0, dx[]={-1,1,0,0}, dy[]={0,0,-1,1};
    while (!q.empty() && fresh>0) {
        int sz=q.size(); mins++;
        for (int i=0;i<sz;i++) {
            auto [x,y]=q.front(); q.pop();
            for (int d=0;d<4;d++) {
                int nx=x+dx[d],ny=y+dy[d];
                if (nx>=0&&nx<n&&ny>=0&&ny<m&&g[nx][ny]==1) {
                    g[nx][ny]=2; fresh--; q.push({nx,ny});
                }
            }
        }
    }
    cout << (fresh==0?mins:-1) << "\n";
}