// Copyright (C) 2017 __debug.

// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; version 3

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program; If not, see <http://www.gnu.org/licenses/>.


#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/priority_queue.hpp>

#define x first
#define y second
#define MP std::make_pair
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define DEBUG(...) fprintf(stderr, __VA_ARGS__)
#ifdef __linux__
#define getchar getchar_unlocked
#define putchar putchar_unlocked
#endif

using std::pair;
using std::vector;
using std::string;

typedef long long LL;
typedef pair<int, int> Pii;

const int oo = 0x3f3f3f3f;

template<typename T> inline bool chkmax(T &a, T b) { return a < b ? a = b, true : false; }
template<typename T> inline bool chkmin(T &a, T b) { return b < a ? a = b, true : false; }
string procStatus()
{
    std::ifstream t("/proc/self/status");
    return string(std::istreambuf_iterator<char>(t), std::istreambuf_iterator<char>());
}
template<typename T> T read(T &x)
{
    int f = 1;
    char ch = getchar();
    for (; !isdigit(ch); ch = getchar())
        f = (ch == '-' ? -1 : 1);
    for (x = 0; isdigit(ch); ch = getchar())
        x = 10 * x + ch - '0';
    return x *= f;
}
template<typename T> void write(T x)
{
    if (x == 0) {
        putchar('0');
        return;
    }
    if (x < 0) {
        putchar('-');
        x = -x;
    }
    static char s[20];
    int top = 0;
    for (; x; x /= 10)
        s[++top] = x % 10 + '0';
    while (top)
        putchar(s[top--]);
}
// EOT

const int MAXN = 2e3 + 5;

int N, M;
char A[MAXN][MAXN];

void input()
{
    read(N); read(M);
    for (int i = 1; i <= N; ++i) {
        scanf("%s", A[i] + 1);
    }
}

void solve()
{
    static bool forbid[MAXN][MAXN];

    for (int i = 1; i < N; ++i) {
        for (int j = 1; j < M; ++j) {
            forbid[i][j] = (A[i][j] == '#') ^ (A[i + 1][j] == '#') ^ (A[i][j + 1] == '#') ^ (A[i + 1][j + 1] == '#');
        }
    }

    static int up[MAXN];
    int ans = std::max(N, M);

    for (int i = 1; i < N; ++i) {
        for (int j = 1; j < M; ++j) {
            up[j] = (forbid[i][j] ? 0 : up[j] + 1);
        }
        static int st[MAXN];
        static int lbnd[MAXN];
        int top = 0;
        for (int j = 1; j < M; ++j) {
            for (; top > 0 && up[st[top]] > up[j]; --top) {
                chkmax(ans, (up[st[top]] + 1) * (j - lbnd[st[top]] + 1));
            }
            lbnd[j] = st[top] + 1;
            st[++top] = j;
        }
        for (; top > 0; --top) {
            chkmax(ans, (up[st[top]] + 1) * (M - lbnd[st[top]] + 1));
        }
    }

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

int main()
{
#ifdef __DEBUG
    freopen("F.in", "r", stdin);
    freopen("F.out", "w", stdout);
#endif

    input();
    solve();
}

// 辛苦遭逢起一经，干戈寥落四周星。
//     -- 文天祥《过零丁洋》


Measured memory_bytes: 8064