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 <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T &a, T b)
{
    if (a < b)
    {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
inline bool chmin(T &a, T b)
{
    if (a > b)
    {
        a = b;
        return 1;
    }
    return 0;
}
typedef long long int ll;

#define EPS (1e-7)
#define INF (1e9)
#define PI (acos(-1))
#define MOD (1000000007)
#define ALL(v) (v).begin(), (v).end()
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

//-------------------------------------

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    while (true)
    {
        int d, w;
        cin >> d >> w;
        if (d == 0 && w == 0)
        {
            return 0;
        }
        vector<vector<int>> e(d, vector<int>(w));
        for (int i = 0; i < d; i++)
        {
            for (int j = 0; j < w; j++)
            {
                cin >> e[i][j];
            }
        }
        int ans = 0;
        for (int sy = 0; sy < d; sy++)
        {
            for (int sx = 0; sx < w; sx++)
            {
                for (int cntx = 2; sx + cntx < w; cntx++)
                {
                    for (int cnty = 2; sy + cnty < d; cnty++)
                    {
                        int gx = sx + cntx;
                        int gy = sy + cnty;
                        // sxからgx、syからgyまでの区間を確認する
                        int kabe = INF; //壁の中で最小を更新
                        int rec = 0;
                        for (int i = sy; i <= gy; i++)
                        {
                            for (int j = sx; j <= gx; j++)
                            {
                                if (i == sy || i == gy || j == sx || j == gx)
                                {
                                    chmin(kabe, e[i][j]);
                                }
                            }
                        }
                        bool flg = false;
                        for (int i = sy + 1; i < gy && !flg; i++)
                        {
                            for (int j = sx + 1; j < gx && !flg; j++)
                            {
                                if (e[i][j] >= kabe)
                                {
                                    rec = 0;
                                    flg = true;
                                }
                                else
                                {
                                    rec += kabe - e[i][j];
                                }
                            }
                        }
                        chmax(ans, rec);
                    }
                }
            }
        }
        cout << ans << endl;
    }
}

C++: 
#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> Ans;
    while (1)
    {
        int d, w;
        cin >> d >> w;
        if (d == 0 && w == 0)
            break;

        vector<vector<int>> heights(d, vector<int>(w));
        for (int i = 0; i < d; i++)
        {
            for (int j = 0; j < w; j++)
            {
                int in;
                cin >> in;
                heights.at(i).at(j) = in;
            }
        }

        int ans = 0;
        for (int x1 = 0; x1 < w; x1++)
        {
            for (int y1 = 0; y1 < d; y1++)
            {
                for (int x2 = x1 + 2; x2 < w; x2++)
                {
                    for (int y2 = y1 + 2; y2 < d; y2++)
                    {
                        int HeightMin = INT_MAX, sum = 0;
                        for (int x = x1; x <= x2; x++)
                        {
                            HeightMin = min(HeightMin, heights[y1][x]);
                            HeightMin = min(HeightMin, heights[y2][x]);
                        }
                        for (int y = y1; y <= y2; y++)
                        {
                            HeightMin = min(HeightMin, heights[y][x1]);
                            HeightMin = min(HeightMin, heights[y][x2]);
                        }
                        for (int x = x1 + 1; x <= x2 - 1; x++)
                        {
                            for (int y = y1 + 1; y <= y2 - 1; y++)
                            {
                                if (heights[y][x] >= HeightMin)
                                    goto Exit;
                                else
                                {
                                    sum += HeightMin - heights[y][x];
                                }
                            }
                        }
                        ans = max(ans, sum);
                    Exit:;
                    }
                }
            }
        }
        Ans.push_back(ans);
    }
    for (int i = 0; i < Ans.size(); i++)
    {
        cout << Ans[i] << endl;
    }
}