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;

int dx[9] = {0, 0, 1, 1, 1, 0, -1, -1, -1}, dy[9] = {0, -1, -1, 0, 1, 1, 1, 0, -1};

int main() {
  int d, w;
  while ( cin >> d >> w, d ) {
    int e[d][w];
    for ( int i = 0; i < d; i++ ) {
      for ( int j = 0; j < w; j++ ) {
	cin >> e[i][j];
      }
    }

    int ans = 0;    
    for ( int a = 1; a <= 9; a++ ) {
      for ( int i = 1; i < d-1; i++ ) {
	for ( int j = 1; j < w-1; j++ ) {
	  if ( e[i][j] >= a ) continue;
	  int y = i, x = j;
	  while ( y+1 < d-1 && e[y+1][j] < a ) y++;
	  while ( x+1 < w-1 && e[i][x+1] < a ) x++;
	  bool flag = true;
	  int sum = 0;
	  // if ( a == 2 ) cout << i << j << y << x << endl;
	  for ( int r = i; r <= y; r++ ) {
	    for ( int c = j; c <= x; c++ ) {
	      sum += a-e[r][c];	      
	      for ( int k = 0; k < 9; k++ ) {	      
		int ny = r+dy[k], nx = c+dx[k];
		if ( k == 0 && e[ny][nx] >= a ) {		  
		  flag = false;
		  r = y; c = x; break;		  
		}
		if ( (ny < i || y < ny || nx < j || x < nx) && e[ny][nx] < a ) {
		  flag = false;
		  r = y; c = x; break;		  
		}
	      }	      
	    }
	  }
	  if ( flag ) {
	    ans = max(ans, sum);
	    // if ( sum == 22 ) cout << i << " " << j << " " << a << endl;
	  }	  
	}
      }
    }

    cout << ans << endl;
  }
  
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
template<class T> inline bool chmin(T& a, T b) {if(a>b){a=b; return true;}return false;}
template<class T> inline bool chmax(T& a, T b) {if(a<b){a=b; return true;}return false;}
int d,w,field[15][15];
int main(){
  while(1){
    int ans=-1;
    cin >> d >> w;
    if(d==0 && w==0) break;
    for(int i=0;i<d;i++)for(int j=0;j<w;j++) cin >> field[j][i];
    for(int i=1;i<=w-2;i++)for(int j=1;j<=d-2;j++)for(int k=1;k<=w-2;k++)for(int l=1;l<=d-2;l++){
      int circuit_min=114514;
      //cout << "(" << i << "," << j<< ")" << endl;
      //cout << "(" << k << "," <<l << ")" << endl;
      for(int x=i-1;x<=k+1;x++)for(int y=j-1;y<=l+1;y++){
        if((i<=x && x<=k) && (j<=y && y<=l));
        else chmin(circuit_min,field[x][y]);
      }
      //cout << "circuit_min:" << circuit_min << endl;
      int in_max = -1;
      for(int x=i;x<=k;x++)for(int y=j;y<=l;y++){
        chmax(in_max,field[x][y]);
        //cout << field[x][y] << endl;
      }
      //cout <<  "in_max:" << in_max << endl;
      if(circuit_min>in_max){
        int sum=0;
        for(int x=i;x<=k;x++)for(int y=j;y<=l;y++) sum+=circuit_min-field[x][y];
        chmax(ans,sum);
      }
      //cout << "ans;" << ans << endl;
    }
    if(ans==-1) cout << "0" << endl;
    else cout << ans << endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(long long int i=0;i<n;++i)
typedef long long int ll;

int func(vector<vector<int>> m,int a,int b,int c,int d){
    int ret=10;
    for(int i=b;i<=d;i++){
        ret=min({ret,m[a][i],m[c][i]});
    }
    for(int i=a;i<=c;i++){
        ret=min({ret,m[i][b],m[i][d]});
    }
    int ff=-1;
    a++;b++;c--;d--;
    for(int i=b;i<=d;i++){
        ff=max({ff,m[a][i],m[c][i]});
    }
    for(int i=a;i<=c;i++){
        ff=max({ff,m[i][b],m[i][d]});
    }
    if(ff>=ret){
        ret=-1;
    }
    return ret;
}

int main(){

    while(1){
        int d,w;
        cin >> d >> w;
        if(d==0)break;
        vector<vector<int>> m(d,vector<int>(w));
        for(int i=0;i<d;i++){
            for(int j=0;j<w;j++){
                cin >> m[i][j];
            }
        }
        int ans=0;
        for(int i=0;i<=d-3;i++){
            for(int j=0;j<=w-3;j++){
                for(int k=i+2;k<d;k++){
                    for(int l=j+2;l<w;l++){
                        int mi=func(m,i,j,k,l);
                        int su=0;
                        if(mi==-1)continue;
                        for(int s=i+1;s<k;s++){
                            for(int t=j+1;t<l;t++){
                                su+=max(0,mi-m[s][t]);
                            }
                        }
                        ans=max(ans,su);
                    }
                }
            }
        }
        cout << ans << endl;
    }

    return 0;
}