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>
#define For(i, a, b) for(int (i)=(int)(a); (i)<(int)(b); ++(i))
#define rFor(i, a, b) for(int (i)=(int)(a)-1; (i)>=(int)(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;
template<class T> bool chmax(T &a, const T &b){if(a<b){a=b; return true;} return false;}
template<class T> bool chmin(T &a, const T &b){if(a>b){a=b; return true;} return false;}
template<class T> T div_floor(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>=0 ? a/b : (a+1)/b-1;
}
template<class T> T div_ceil(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>0 ? (a-1)/b+1 : a/b;
}

constexpr lint mod = 1000000007;
constexpr lint INF = mod * mod;
constexpr int MAX = 200010;

int h, w;

void solve(){
    int a[h][w];
    rep(i, h)rep(j, w) scanf("%d", &a[i][j]);
    int ans = 0;
    rep(u, h)For(d, u + 2, h)rep(l, w)For(r, l + 2, w){
        int min_h = mod;
        For(i, u, d + 1){
            chmin(min_h, a[i][l]);
            chmin(min_h, a[i][r]);
        }
        For(j, l, r + 1){
            chmin(min_h, a[u][j]);
            chmin(min_h, a[d][j]);
        }

        int tmp = 0;
        For(x, u + 1, d){
            For(y, l + 1, r){
                if(a[x][y] >= min_h){
                    tmp = -1;
                    break;
                }
                tmp += min_h - a[x][y];
            }
            if(tmp < 0) break;
        }
        chmax(ans, tmp);
    }
    printf("%d\n", ans);
}

int main(){
    while(scanf("%d%d", &h, &w) && h){
        solve();
    }
}

C++: #include<iostream>

int garden[10][10] = {};
int min = 10;

int pond_area(int sx,int sy,int width,int height){
    int area = 0;
    for(int i=sx+1;i<width+sx-1;i++){
        for(int j=sy+1;j<height+sy-1;j++){
            int d = garden[j][i];
            area += (min - d);
        }
    }
    return area;
}
    
bool isPond(int sx,int sy,int width,int height){
    //std::cout << "start:(" << sx << ',' << sy << ") width = " << width << " height=" << height << '\n';
    int max = 0;
    min = 10;
    for(int k=sy;k<height+sy;k++){
        int d1 = garden[k][sx];
        int d2 = garden[k][sx+width-1];
        if(d1>d2) d1 = d2;
        if(min>d1) min = d1;
    }
    for(int k=sx;k<width+sx;k++){
        int d1 = garden[sy][k];
        int d2 = garden[sy+height-1][k];
        if(d1>d2) d1=d2;
        if(min>d1) min = d1;
    }
    
    for(int i=sx+1;i<width+sx-1;i++){
        for(int j=sy+1;j<height+sy-1;j++){
            int d = garden[j][i];
            if(max<d) max = d;
        }
    }
    
    //std::cout << "  min=" << min << "  max=" << max << '\n';
    
    if(min>max) return true;
    else return false;
}

    
int main(){
    int m,n;
    int max = 0;
    std::cin >> m >> n;
    for(int i=0;i<m;i++){
        for(int j=0;j<n;j++){
            std::cin >> garden[i][j];
        }
    }
    /*for(int i=0;i<m;i++){
        for(int j=0;j<n;j++){
            std::cout << garden[i][j] << ' ';
        }
        std::cout << '\n';
    }
     */
    for(int height=m;height>2;height--){
        for(int width=n;width>2;width--){
            for(int sy=0;sy<m-height+1;sy++){
                for(int sx=0;sx<n-width+1;sx++){
                    if(isPond(sx,sy,width,height)){
                        int area = pond_area(sx,sy,width,height);
                        if(area > max) max = area;
                        //std::cout << "area=" << area << '\n';
                    }
                }
            }
        }
    }
    std::cout << max << '\n';
    return 0;
}