Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func main() {
	io := NewIo()
	defer io.Flush()
	for {
		d, w := io.NextInt(), io.NextInt()
		if d == 0 {
			break
		}
		e := make([][]int, d)
		for i := 0; i < d; i++ {
			e[i] = make([]int, w)
			for j := 0; j < w; j++ {
				e[i][j] = io.NextInt()
			}
		}
		ans := 0
		for i := 0; i < d-2; i++ {
			for j := 0; j < w-2; j++ {
				for k := i + 2; k < d; k++ {
					for l := j + 2; l < w; l++ {
						ans = intMax(vol(e, i, j, k, l), ans)
					}
				}
			}
		}
		io.Println(ans)
	}
}

func vol(e [][]int, a, b, c, d int) int {
	outer := 10
	for i := a; i <= c; i++ {
		outer = intMin(e[i][b], outer)
		outer = intMin(e[i][d], outer)
	}
	for j := b; j <= d; j++ {
		outer = intMin(e[a][j], outer)
		outer = intMin(e[c][j], outer)
	}
	a++
	b++
	vol := 0
	for i := a; i < c; i++ {
		for j := b; j < d; j++ {
			if e[i][j] >= outer {
				return 0
			}
			vol += outer - e[i][j]
		}
	}
	return vol
}

func intMax(a, b int) int {
	if a < b {
		return b
	}
	return a
}

func intMin(a, b int) int {
	if a > b {
		return b
	}
	return a
}

// I/O template
type Io struct {
	reader    *bufio.Reader
	writer    *bufio.Writer
	tokens    []string
	nextToken int
}

func NewIo() *Io {
	return &Io{
		reader: bufio.NewReader(os.Stdin),
		writer: bufio.NewWriter(os.Stdout),
	}
}

func (io *Io) Flush() {
	err := io.writer.Flush()
	if err != nil {
		panic(err)
	}
}

func (io *Io) NextLine() string {
	var buff []byte
	for {
		line, isPrefix, err := io.reader.ReadLine()
		if err != nil {
			panic(err)
		}
		buff = append(buff, line...)
		if !isPrefix {
			break
		}
	}
	return string(buff)
}

func (io *Io) Next() string {
	for io.nextToken >= len(io.tokens) {
		line := io.NextLine()
		io.tokens = strings.Fields(line)
		io.nextToken = 0
	}
	s := io.tokens[io.nextToken]
	io.nextToken++
	return s
}

func (io *Io) NextInt() int {
	i, err := strconv.Atoi(io.Next())
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) NextFloat() float64 {
	i, err := strconv.ParseFloat(io.Next(), 64)
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) Println(a ...interface{}) {
	fmt.Fprintln(io.writer, a...)
}

func (io *Io) Printf(format string, a ...interface{}) {
	fmt.Fprintf(io.writer, format, a...)
}

func (io *Io) PrintIntSlice(a []int) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.Println(b...)
}

func (io *Io) PrintStringSlice(a []string) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.Println(b...)
}

func Log(name string, value interface{}) {
	fmt.Fprintf(os.Stderr, "%s=%+v\n", name, value)
}

func ExampleIo() {
	io := NewIo()
	defer io.Flush()
	a := make([]int, 0, 3)
	a[0], a[1], a[2] = io.NextInt(), io.NextInt(), io.NextInt()
	io.Println(a)
	io.PrintIntSlice(a)
	Log("io", a)
}

C++: #include<iostream>
#include<algorithm>
#include<vector>

using namespace std;

struct Pond
{
  int up,down,left,right;
  vector<int> canal;
  vector<int> moat;
  int min,max;
  void make(int up,int down,int left,int right,vector<int> canal,vector<int>moat)
  {
    this->up=up; this->down=down; this->left=left; this->right=right;
    this->canal=canal; this->moat=moat;
  }
  void setVals(){
    max= *max_element(canal.begin(),canal.end());
    min= *min_element(moat.begin(),moat.end());
  }
  bool isMake()
  {
    return (max<min);
  }
};

int calc(Pond pond)
{
  int sum=0;
  for(int i=0;i<pond.canal.size();i++){
    sum+=pond.min-pond.canal[i];
  }
  return sum;
}
void print(vector<int> list){
  for(int x:list) cout<<x<<" ";
  cout<<endl;
}
int main()
{
  while(1){
    int depth,width;
    vector<int> canal(0),moat(0);
    cin>>depth>>width;
    int stage[depth][width];
    if((depth==0)and(width==0)) return 0;
  
    for(int i=0;i<depth;i++){
      for(int j=0;j<width;j++){
	cin>>stage[i][j];
      }
    }
    int mx=0;
    for(int up=0;up<depth-1;up++){
      for(int left=0;left<width-1;left++){
	for(int down=up+2;down<depth;down++){
	  for(int right=left+2;right<width;right++){
	    //cout<<"up="<<up<<" down="<<down<<endl;
	    //cout<<"left="<<left<<" right="<<right<<endl;
	    canal.resize(0);
	    moat.resize(0);
	    for(int s=up;s<=down;s++){
	      for(int t=left;t<=right;t++){
		//cout<<"s="<<s<<" t="<<t<<endl;
		if((s!=up and s!=down)and(t!=left and t!=right)){
		  //cout<<"canal.push("<<stage[s][t]<<")"<<endl;
		  canal.push_back(stage[s][t]);
		}else{
		  //cout<<"moat.push("<<stage[s][t]<<")"<<endl;
		  moat.push_back(stage[s][t]);
		}
	      }
	    }
	    //print(canal);
	    //print(moat);
	    Pond pond;
	    pond.make(up,down,left,right,canal,moat);
	    pond.setVals();
	    //cout<<"make pond:"<<endl;
	    if(pond.isMake()){
	      //cout<<"judge=YES"<<endl;
	      mx=max(mx,calc(pond));
	    }else{
	      //cout<<"judge=NO"<<endl;
	    }
	  }
	}
      }
    }
    //cout<<"count="<<count<<endl;
    cout<<mx<<endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
#define int long long
#define endl '\n'
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define REP(i, n) FOR(i, 0, n)
using namespace std;

int d, w;
int e[10][10];

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    while (cin >> d >> w, d) {
        REP (i, d) REP (j, w) cin >> e[i][j];
        int ans = 0;
        REP (i, d) REP (j, w) FOR (k, i + 2, d) FOR (l, j + 2, w) {
            int minh = 10;
            FOR (y, i, k + 1) minh = min({minh, e[y][j], e[y][l]});
            FOR (x, j, l + 1) minh = min({minh, e[i][x], e[k][x]});
            int sum = 0, ok = 1;
            FOR (y, i + 1, k) FOR (x, j + 1, l) {
                sum += minh - e[y][x];
                if (e[y][x] >= minh) ok = 0;
            }
            if (ok) ans = max(ans, sum);
        }
        cout << ans << endl;
    }
}

C++: #include "bits/stdc++.h"

using namespace std;

using ll = long long;
using ld = long double;
using P = pair<int, int>;
constexpr ld EPS = 1e-12;
constexpr int INF = numeric_limits<int>::max() / 2;
constexpr int MOD = 1e9 + 7;

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int H,W;
    while(cin>>H>>W, H || W){
        vector<vector<int>> e(H,vector<int>(W));
        for(int i=0;i<H;i++){
            for(int j=0;j<W;j++){
                cin>>e[i][j];
            }
        }
        int res=0;
        for(int h=1;h<=H-2;h++){
            for(int w=1;w<=W-2;w++){
                for(int sx=1;sx<=H-1-h;sx++){
                    for(int sy=1;sy<=W-1-w;sy++){
                        int hmin=INF, pmax=0;
                        for(int i=sx-1;i<=sx+h;i++){
                            for(int j=sy-1;j<=sy+w;j++){
                                if(sx<=i && i<=sx+h-1 && sy<=j && j<=sy+w-1){
                                    pmax=max(pmax, e[i][j]);
                                }
                                else{
                                    hmin=min(hmin, e[i][j]);
                                }
                            }
                        }
                        if(hmin<=pmax) continue;
                        int sum = 0;
                        for(int i=sx;i<=sx+h-1;i++){
                            for(int j=sy;j<=sy+w-1;j++){
                                sum += hmin-e[i][j];
                            }
                        }
                        res=max(res,sum);
                    }
                }
            }
        }
        cout<<res<<endl;
    }
}