//#pragma GCC optimize("O3,unroll-loops")
//#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <bits/stdc++.h>
#define pb push_back
#define ll long long
#define F first
#define S second
#define all(v) (v).begin(),(v).end()
#define ull unsigned long long
#define ahahahant ios_base::sync_with_stdio(0);cin.tie(0);
#define int long long
using namespace std;
//mt19937 rng( chrono::steady_clock::now().time_since_epoch().count());
//
//ll rand( ll l, ll r )
//{
// uniform_int_distribution <ll> uid( l, r );
// return uid( rng );
//}
ll lcm(ll x, ll y){
return x / __gcd(x,y) * y;
}
ll binpow(ll a, ll b, ll m){
if(!b) return 1ll;
if(b & 1) return binpow(a,b-1,m) * a % m;
int res = binpow(a,b/2,m) % m;
return res * res % m;
}
const int maxn = 1e6 + 6;
const int mod = 1e9 + 7;
const int mod2 = 998244353;
const ll inf = 1e18;
const int inf2 = 1e9;
const int K = 300;
int n,m,a[1005][1005],c[1005][1005],timer;
bool used[1005][1005];
int cnt, mxx, mnn;
void dfs(int x, int y){
cnt++;
used[x][y] = 1;
mxx = max(mxx, a[x][y]);
mnn = min(mnn, a[x][y]);
if(x + 1 <= n && !used[x + 1][y]) dfs(x + 1,y);
if(x - 1 >= 1 && !used[x - 1][y]) dfs(x - 1,y);
if(y + 1 <= m && !used[x][y + 1]) dfs(x,y + 1);
if(y - 1 >= 1 && !used[x][y - 1]) dfs(x,y - 1);
}
void who(){
cin >> n >> m;
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
cin >> a[i][j];
}
}
if(n == 1){
int ans = -1;
for(int i = 1; i <= m; i++){
int mx = 0, mn = 1e9, sz = 0;
for(int j = i; j <= m; j++){
mx = max(mx, a[1][j]);
mn = min(mn, a[1][j]);
sz++;
ans = max(ans, mx - mn - sz);
}
}
cout << ans;
}
else if(n * m <= 20){
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
c[i][j] = timer++;
}
}
int ans = -1;
for(int mask = 0; mask < (1 << (n * m)) - 1; mask++){
pair <int,int> s;
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
if(mask & (1 << c[i][j])){
used[i][j] = 1;
}
else {
s.F = i;
s.S = j;
}
}
}
cnt = 0;
mxx = 0;
mnn = 1e9;
dfs(s.F,s.S);
ans = max(ans, mxx - mnn - cnt);
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
used[i][j] = 0;
}
}
}
cout << ans << '\n';
}
}
signed main(){
// freopen("yinyang.in", "r", stdin);
// freopen("yinyang.out", "w", stdout);
ahahahant
int tt = 1;
// cin >> tt;
while(tt--){
who();
}
return 0;
}
/*
__builtin_popcount
__builtin_clz
__builtin_ctz
__builtin_parity
int get(int r){
int res = 0;
for(; r >= 0; r = (r & (r + 1)) - 1) res += f[r];
return res;
}
void upd(int i, int val){
for(; i <= n; i = (i | (i + 1))) f[i] += val;
}
*/
