//Zory-2020
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
// typedef __int128 ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
#define FR first
#define SE second
#define MP make_pair
#define PB push_back
#define vc vector
#define db double
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define bin(x) (1ll<<(x))
#define fo(i,l,r) for(int i=(l),I=(r);i<=I;i++)
#define fd(i,r,l) for(int i=(r),I=(l);i>=I;i--)
#define mem(x,val) memset(x,val,sizeof x)
#define Swap(a,b,n) for(int I=0;I<=n;I++) swap(a[I],b[I])
#define PC __builtin_popcountll
#ifdef DEBUG
	#define debug(A,args...) fprintf(stderr,A,##args)
#else
	#define debug(A,args...) printf("")
#endif
#define deb debug("line %d\n",__LINE__)
namespace mine
{
	ll qread()
	{
		ll ans=0,f=1;char c=getchar();
		while(c<'0' or c>'9') {if(c=='-')f=-1;c=getchar();}
		while('0'<=c and c<='9') ans=ans*10+c-'0',c=getchar();
		return ans*f;
	}
	void write(ll num){if(num<0) putchar('-'),num=-num;if(num>=10) write(num/10);putchar('0'+num%10);}
	void write1(ll num){write(num);putchar(' ');}
	void write2(ll num){write(num);putchar('\n');}
	template<typename T>inline bool chmax(T&a,const T&b){return a<b?a=b,1:0;}
	template<typename T>inline bool chmin(T&a,const T&b){return a>b?a=b,1:0;}
	ll gcd(ll x,ll y){return y?gcd(y,x%y):x;}
	bool IN(ll x,ll l,ll r){return l<=x and x<=r;}
	void GG(){puts("-1");exit(0);}

	const db eps=1e-8;
	const int INF=0x3f3f3f3f;
	const int MOD=1e9+7;
	int mm(const int x){return x>=MOD?x-MOD:x;}
	template<typename T> void add(T &x,const int &y){x=(x+y>=MOD?x+y-MOD:x+y);}
	ll qpower(ll x,ll e,int mod=MOD){ll ans=1;while(e){if(e&1)ans=ans*x%mod;x=x*x%mod;e>>=1;}return ans;}
	ll invm(ll x){return qpower(x,MOD-2);}
	const int MM=5e6+10;
	ll fac[MM],facinv[MM],Inv[MM];ll Comb(int n,int m){return n<0 or n<m?0:fac[n]*facinv[m]%MOD*facinv[n-m]%MOD;}
	void PRE()
	{
		fac[0]=1;fo(i,1,MM-1) fac[i]=fac[i-1]*i%MOD;
		facinv[MM-1]=invm(fac[MM-1]);fd(i,MM-1,1) facinv[i-1]=facinv[i]*i%MOD;
		Inv[1]=1;fo(i,2,MM-1) Inv[i]=(MOD-MOD/i)*Inv[MOD%i]%MOD;
	}

	const int N=2e3+10;
	int mxsiz;
    struct DSU
    {
        int fa[N],siz[N];void clear(){ fo(i,0,N-1) fa[i]=i,siz[i]=1; }
        int findfa(int x){return fa[x]==x?x:fa[x]=findfa(fa[x]);}
        void merg(int x,int y){x=findfa(x),y=findfa(y);if(x==y)return;if(siz[x]>siz[y])swap(x,y);fa[x]=y,siz[y]+=siz[x];chmax(mxsiz,siz[y]);}
    }dsu;
	//------------------FIXED------------------
	char mp[N][N];queue<int> dif[N][2][2];
	int b[N];
	void main()
	{
		int n=qread(),m=qread();fo(i,1,n) scanf("%s",mp[i]+1);int ans=n;
		fo(j,1,m-1) fo(x,0,1) fo(y,0,1) {fo(i,1,n) if(((mp[i][j]=='#')^x)!=((mp[i][j+1]=='#')^y)) dif[j][x][y].push(i);dif[j][x][y].push(n+1);}
		fo(ii,1,n) fo(op,0,1)
		{
			fo(j,1,m) b[j]=(mp[ii][j]=='#')^op^1;
			dsu.clear();vc<pii> pp;
			fo(j,1,m-1)
			{
				while(dif[j][b[j]][b[j+1]].front()<=ii) dif[j][b[j]][b[j+1]].pop();
				pp.PB({dif[j][b[j]][b[j+1]].front(),j});
			}sort(all(pp));reverse(all(pp));
			mxsiz=1;for(auto hh:pp) dsu.merg(hh.SE,hh.SE+1),chmax(ans,(hh.FR-ii)*mxsiz);
		}write(ans);
 	}
};//变量重名！
signed main()
{
	#ifdef DEBUG
	//freopen("a.in","r",stdin);
	freopen("z.txt","r",stdin);
	//freopen("a.out","w",stdout);
	#endif
	srand(time(0));
	mine::main();
	debug("\n------------------------------------------\nTime: %.2lf s\n",1.0*clock()/CLOCKS_PER_SEC);
}

Measured memory_bytes: 43392