#pragma comment(linker, "/STACK:1024000000,1024000000")
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <limits.h>
#include <assert.h>
#include <math.h>
#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <string>
#include <bitset>
#include <vector>
using namespace std;

#define LL long long

#define fi first
#define se second
#define lson l,mid,id<<1
#define rson mid+1,r,id<<1|1
#define ls id<<1
#define rs id<<1|1
#define MID(a,b) (((a)+(b))>>1)
#define maxx(a,b) ((a)<(b)?(b):(a))
#define minx(a,b) ((a)<(b)?(a):(b))
#define absx(a) ((a)<0?-(a):(a))
#define mk(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define itr iterator
#define lowbit(x) ((x)&-(x))

typedef unsigned LL ULL;
typedef unsigned uint;
typedef map<int,int> mii;
typedef pair<int,int> pii;
typedef pair<double,double> pdd;
typedef pair<LL,LL> pLL;

template< typename T > inline void read(T &x) {
    x=0; bool f=0; char ch=getchar();
    while(ch<'0' || ch>'9') {if(ch=='-') f=1; ch=getchar();}
    while(ch>='0' && ch<='9') {x=x*10+ch-'0'; ch=getchar();}
    if(f) x=-x;
}

template< typename T > inline void Max(T &a, T b) {if(a<b) a=b;}
template< typename T > inline void Min(T &a, T b) {if(b<a) a=b;}
template< typename T > inline T Abs(T a) {if(a<0) return -a; else return a;}

const double pi=(double) acos(-1.0);
const int MOD=(int) 1e9+7;
const int INF=(int) 0x3f3f3f3f;
const LL  LINF=(LL) INF<<32|INF;
const int SINF=(uint) ~0>>1;
const LL  SLINF=(ULL) (-1)>>1;
const double DINF=(double) 1e50;
const double eps=(double) 1e-8;
const int maxn=(int) 2e3+20;
const int maxm=(int) 1e6+20;
const int maxk=(int) 5e2+20;

inline int sig(double x) {return x<-eps?-1:x>eps;}

//--------------start------------------

int n,m;
char ss[maxn][maxn];
int a[maxn][maxn],b[maxn][maxn];
int top[maxn][maxn];
int stk[maxn],tp;
int high[maxn],L[maxn],R[maxn];


void work()
{
    while(~scanf("%d%d",&n,&m))
    {
        for(int i=1;i<=n;i++) scanf("%s",ss[i]+1);

        for(int i=1;i<=n;i++)
            for(int j=1;j<=m;j++)
                a[i][j] = ss[i][j]=='#';

        for(int i=1;i<=n-1;i++)
            for(int j=1;j<=m-1;j++)
                b[i][j] = a[i][j]^a[i+1][j]^a[i][j+1]^a[i+1][j+1];

        for(int j=1;j<=m-1;j++) top[n][j]=n;
        for(int i=n-1;i>=1;i--)
            for(int j=1;j<=m-1;j++)
                top[i][j] = b[i][j] ? i : top[i+1][j] ;

        LL ans=0;
        for(int i=1,ni;i<=n-1;i=ni+1)
        {
            ni=INF;
            for(int j=1;j<=m-1;j++) high[j]=top[i][j]-i+1, Min(ni,top[i][j]);

            tp=0;
            for(int j=1;j<=m-1;j++)
            {
                while(tp && high[stk[tp]]>=high[j]) tp--;
                L[j] = tp ? stk[tp]+1 : 1 ;
                stk[++tp]=j;
            }

            tp=0;
            for(int j=m-1;j>=1;j--)
            {
                while(tp && high[stk[tp]]>=high[j]) tp--;
                R[j] = tp ? stk[tp] : m ;
                stk[++tp]=j;
            }

            for(int j=1;j<=m-1;j++) Max(ans,(LL)high[j]*(R[j]-L[j]+1));
        }

        Max(ans,(LL)max(n,m));

        printf("%lld\n",ans);
    }
}

//--------------end--------------------

int main()
{
#ifdef yukihana0416
freopen("in.txt","r",stdin);
//freopen("out.txt","w",stdout);
#endif // yukihana0416
    work();
    return 0;
}



Measured memory_bytes: 52096