#include "bits/stdc++.h"
using namespace std;
int main(){
ios::sync_with_stdio(false); cin.tie(nullptr);
int n, m;
cin >> n >> m;
vector <vector <int>> a(n+1, vector <int> (m+1));
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
cin >> a[i][j];
}
}
vector <vector <int>> dp(n+2, vector <int> (m+2, 1e9));
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
dp[i][j] = min(dp[i][j], min(a[i][j], dp[i][j-1] + 1));
}
for(int j = m; j >= 1; j--){
dp[i][j] = min(dp[i][j], min(a[i][j], dp[i][j+1] + 1));
}
}
for(int j = 1; j <= m; j++){
for(int i = 1; i <= n; i++){
dp[i][j] = min(dp[i][j], dp[i-1][j] + 1);
}
for(int i = n; i >= 1; i--){
dp[i][j] = min(dp[i][j], dp[i+1][j] + 1);
}
}
int ans = -1;
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
ans = max(ans, a[i][j] - dp[i][j]);
}
}
cout << ans-1;
}
