///////////////////////////////////////////////////////////////////////////////

#include <bits/stdc++.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>

using namespace std;

///////////////////////////////////////////////////////////////////////////////

#define DEBUG 0

#define pb push_back
#define V vector
#define M unordered_map
#define S static

#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG

typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;

///////////////////////////////////////////////////////////////////////////////

template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}

template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

template <typename TT, typename T>
void incr_m(map<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

void llin(ll &a)
{
    CIN(a);
}

void llinl1(V<ll> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}

void llinl2(V<t2> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}

void llinl3(V<t3> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}

void llinl4(V<t4> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}

void llina(V<ll> &v, ll count)
{
    llinl1(v, count);
}

template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}

template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}

template <typename T>
void _debug_print(T x)
{
    cout << x << endl;
}

void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}

void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}

void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}

template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}

template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

template <typename TT, typename T>
void _debug_print(const map<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
#if DEBUG
    struct tm *tm = localtime(&tv.tv_sec);
#endif
    debug_printf("-- %s %02d:%02d:%02d.%06ld\n",
                 prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}

///////////////////////////////////////////////////////////////////////////////

void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif

    _main();
    return 0;
}

void _main()
{
    ll h; llin(h);
    ll w; llin(w);
    V<string> grid;
    rep (i, h) {
        string tmp; CIN(tmp);
        grid.pb(tmp);
    }

    S ll hs[4] = {0, 0, 1, 1};
    S ll ws[4] = {0, 1, 0, 1};
    S bool ngs[2010][2010];
    memset(ngs, 0, sizeof(ngs));
    rep (hh, h-1) rep (ww, w-1) {
        ll cnt = 0;
        rep (i, 4) cnt += (grid[hh+hs[i]][ww+ws[i]] == '#');
        if (cnt & 1LL) ngs[hh][ww] = true;
    }

#if DEBUG
    debug_printf("---- ngs\n");
    rep (hh, h-1) {
        rep (ww, w-1) {
            debug_printf("%d ", ngs[hh][ww]);
        }
        debug_printf("\n");
    }
#endif // DEBUG

    S V<ll> ptss_wh[2010];
    rep (hh, h) rep (ww, w) if (ngs[hh][ww]) ptss_wh[ww].pb(hh);

    S V<ll> wait_w[2010][2010];
    rep (ww, w) {
        ll idx = 0;
        rep (hs, h) {
            if (idx >= ptss_wh[ww].size()) break;
            ll hh = ptss_wh[ww][idx];
            if (hs > hh) idx++;
            if (idx >= ptss_wh[ww].size()) break;
            hh = ptss_wh[ww][idx];
            wait_w[hs][hh].pb(ww);
        }
    }

    rep (hs, h) srep (hh, hs, h) {
        debug_printf("---- wait_w %lld %lld\n", hs, hh);
        debug_print(wait_w[hs][hh]);
    }

#if 0
    rep (hs, 0) {
        ll idx = 0;
        srep (hh, hs, h) {
            ll h0 = ptss_wh[hh][idx];
            wait_w[h0][hh].pb(ww);
            for (hh : ptss_wh[ww]) {
                ll h0 = ptss_wh[ww][i];
                ll h1 = ptss_wh[ww][i+1];
                srep (hh, h0, h1) pts_hw_wait[hh][].pb(ww);
            }
#if 0
            auto iter = lower_bound(ALL(ptss_wh[ww]), hs);
            if (iter == ptss_wh[ww].end()) continue;
            pts_hw_wait[*iter].pb(ww);
#endif
        }
    }
#endif

    ll ans = 0;
    set<ll> ptsset_orig; ptsset_orig.insert(-1); ptsset_orig.insert(w-1);
    set<ll> m_lenset_orig; m_lenset_orig.insert(-w);
    ll lencnts[2010] = {};

    rep (hs, h) {
        set<ll> ptsset = ptsset_orig;
        set<ll> m_lenset = m_lenset_orig;
        memset(lencnts, 0, sizeof(lencnts));
        lencnts[w] = 1;

        srep (he, hs+1, h+1) {
            ll len = -*(m_lenset.begin());
            ans = max(ans, (he-hs) * len);

            for (auto ww : wait_w[hs][he-1]) {
                auto iter = ptsset.lower_bound(ww);
                ll w2 = *iter;
                iter--;
                ll w1 = *iter;

                lencnts[w2-w1]--; if (!lencnts[w2-w1]) m_lenset.erase(-(w2-w1));
                ptsset.insert(ww);
                lencnts[ww-w1]++; m_lenset.insert(-(ww-w1));
                lencnts[w2-ww]++; m_lenset.insert(-(w2-ww));
            }
        }
    }

    debug_printf("---- ans\n");
    cout << ans << endl;
}

///////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 173596