#include "bits/stdc++.h"
using namespace std;
#define SZ(s) (int)s.size()
#define ff first
#define ss second
#define int long long
const int N = 1e3 + 5;
const int M = 998244353;
int T, n, m, a[N][N], mp1[N][N], b[N], vis[N][N], ans, mn, mx, sz;
pair<int,int> mp[N];
void dfs(int x){
int c = mp[x].ff, d = mp[x].ss;
mn = min(mn, a[c][d]);
mx = max(mx, a[c][d]);
sz++;
vis[c][d] = 1;
if(c > 1 and !vis[c-1][d] and b[mp1[c-1][d]]) dfs(mp1[c-1][d]);
if(d > 1 and !vis[c][d-1] and b[mp1[c][d-1]]) dfs(mp1[c][d-1]);
if(c < n and !vis[c+1][d] and b[mp1[c+1][d]]) dfs(mp1[c+1][d]);
if(d < m and !vis[c][d+1] and b[mp1[c][d+1]]) dfs(mp1[c][d+1]);
}
void f(int x){
if(x == n*m+1){
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
vis[i][j] = 0;
}
}
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
if(!vis[i][j] and b[mp1[i][j]]){
sz = mx = 0;
mn = 1e9;
dfs(mp1[i][j]);
ans = max(ans, mx-mn-sz);
}
}
}
return;
}
b[x] = 1;
f(x+1);
b[x] = 0;
f(x+1);
}
signed main(){
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n >> m;
int cnt = 0;
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
cin >> a[i][j];
mp[++cnt] = {i,j};
mp1[i][j] = cnt;
}
}
f(1);
cout << ans << '\n';
return 0;
}
