Problem:
Score : 700 points

Problem Statement

We have a board with an H \times W grid.
Each square in the grid is painted in black or white. The square at the i-th row from the top and j-th column from the left is black if the j-th character in S_i is #, and white if that character is ..

Snuke can perform the following operation on the grid any number of times:

Select a row or column in the grid, and invert the color of all the squares in that row or column (that is, black squares become white and vice versa).

Then, Snuke draws a rectangle along grid lines. Here, all the squares contained in the rectangle must be painted in black.

Find the maximum possible area of Snuke's rectangle when the operation is performed optimally.

Constraints

2 \leq H \leq 2000

2 \leq W \leq 2000

|S_i| = W

S_i consists of # and ..

Input

Input is given from Standard Input in the following format:

H W
S_1
S_2
:
S_H

Output

Print the maximum possible area of Snuke's rectangle.

Sample Input 1

3 3
..#
##.
.#.

Sample Output 1

6

If the first row from the top and the third column from the left are inverted, a 2 \times 3 rectangle can be drawn, as shown below:

Sample Input 2

4 4
....
....
....
....

Sample Output 2

16

Sample Input 3

10 8
##...#.#
##...#.#
..###.#.
#.##.#.#
.#..#.#.
..##.#.#
##.#.#..
...#.#..
###.#.##
###..###

Sample Output 3

27

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.function.IntUnaryOperator;
import java.util.function.LongUnaryOperator;


public class Main {
    public static void main(String[] args) {
        StringBuilder out = new StringBuilder();
        solve(out);
        PrintWriter pw = new PrintWriter(System.out);
        pw.println(out);
        pw.flush();
        pw.close();
    }

    public static void solve(StringBuilder out) {
        int h = In.ni();
        int w = In.ni();
        boolean[][] g = In.grid(h, w, '#');
        for (int i = h - 1; i > 0; i--) for (int j = 0; j < w; j++) {
            g[i][j] = !(g[i][j] ^ g[i - 1][j]);
        }
        for (int j = 0; j < w; j++) g[0][j] = true;
        for (int j = w - 1; j > 0; j--) for (int i = 0; i < h; i++) {
            g[i][j] = !(g[i][j] ^ g[i][j - 1]);
        }
        for (int i = 0; i < h; i++) g[i][0] = false;
        int[][] hst = new int[h][w];
        for (int j = 0; j < w; j++) hst[h - 1][j] = g[h - 1][j] ? 1 : 0;
        for (int i = h - 2; i >= 0; i--) for (int j = 0; j < w; j++) {
            hst[i][j] = g[i][j] ? hst[i + 1][j] + 1 : 0;
        }
        int max = 0;
        for (int i = 0; i < h; i++) {
            if (i < h - 1) {
                for (int j = 1; j < w; j++) hst[i][j] = hst[i + 1][j] + 1;
            }
            max = Math.max(max, histRectMax(hst[i]));
        }
        int m = Math.max(h, w);
        out.append(Math.max(m, max));
    }

    public static int histRectMax(int[] h) {
        final int n = h.length;
        Stack<int[]> s = new Stack<>();
        int max = 0;
        for (int j = 0; j < n; j++) {
            int hj = h[j];
            if (s.size() == 0) {
                s.add(new int[]{j, hj});
            } else {
                int[] top = s.peek();
                if (top[1] < hj) {
                    s.add(new int[]{j, hj});
                } else if (top[1] > hj) {
                    while (s.size() > 0 && s.peek()[1] >= hj) {
                        top = s.poll();
                        max = Math.max(max, top[1] * (j - top[0] + 1));
                    }
                    s.add(new int[]{top[0], hj});
                }
            }
        }
        int[] top;
        while (s.size() > 0) {
            top = s.poll();
            max = Math.max(max, top[1] * (n - top[0] + 1));
        }
        return max;
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 */
final class In {
    public static final FastScanner fsc = new FastScanner();
    public static int ni() {return fsc.nextInt();}
    public static int[] ni(final int n) {
        final int[] a = new int[n];
        for (int i = 0; i < n; i++) a[i] = fsc.nextInt();
        return a;
    }
    public static int[] ni(final int n, final IntUnaryOperator f) {
        final int[] a = new int[n];
        for (int i = 0; i < n; i++) a[i] = f.applyAsInt(fsc.nextInt());
        return a;
    }
    public static int[][] ni(final int n, final int m) {
        final int[][] a = new int[n][m];
        for (int i = 0; i < n; i++) a[i] = ni(m);
        return a;
    }
    public static int[][] ni(final int n, final int m, final IntUnaryOperator f) {
        final int[][] a = new int[n][m];
        for (int i = 0; i < n; i++) a[i] = ni(m, f);
        return a;
    }
    public static long nl() {return fsc.nextLong();}
    public static long[] nl(final int n) {
        final long[] a = new long[n];
        for (int i = 0; i < n; i++) a[i] = fsc.nextLong();
        return a;
    }
    public static long[] nl(final int n, final LongUnaryOperator f) {
        final long[] a = new long[n];
        for (int i = 0; i < n; i++) a[i] = f.applyAsLong(fsc.nextLong());
        return a;
    }
    public static long[][] nl(final int n, final int m) {
        final long[][] a = new long[n][m];
        for (int i = 0; i < n; i++) a[i] = nl(m);
        return a;
    }
    public static long[][] nl(final int n, final int m, final LongUnaryOperator f) {
        final long[][] a = new long[n][m];
        for (int i = 0; i < n; i++) a[i] = nl(m, f);
        return a;
    }
    public static char[] nc() {return fsc.next().toCharArray();}
    public static char[][] nc(final int n) {
        final char[][] c = new char[n][];
        for (int i = 0; i < n; i++) c[i] = nc();
        return c;
    }
    public static double nd() {return fsc.nextDouble();}
    public static double[] nd(final int n) {
        final double[] a = new double[n];
        for (int i = 0; i < n; i++) a[i] = fsc.nextDouble();
        return a;
    }
    public static double[][] nd(final int n, final int m) {
        final double[][] a = new double[n][m];
        for (int i = 0; i < n; i++) a[i] = nd(m);
        return a;
    }
    public static String ns() {return fsc.next();}
    public static String[] ns(final int n) {
        final String[] s = new String[n];
        for (int i = 0; i < n; i++) s[i] = fsc.next();
        return s;
    }
    public static boolean[][] grid(final int h, final int w, final char trueCharacter) {
        final boolean[][] grid = new boolean[h][w];
        for (int i = 0; i < h; i++) {
            final char[] s = fsc.next().toCharArray();
            for (int j = 0; j < w; j++) grid[i][j] = s[j] == trueCharacter;
        }
        return grid;
    }
}


final class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) return true;
        ptr = 0;
        try {buflen = in.read(buffer);}
        catch (final IOException e) {e.printStackTrace();}
        return buflen > 0;
    }
    private int readByte() {return hasNextByte() ? buffer[ptr++] : -1;}
    public boolean hasNext() {
        while (hasNextByte() && !(33 <= buffer[ptr] && buffer[ptr] <= 126)) ptr++;
        return hasNextByte();
    }
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        final StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (33 <= b && b <= 126) {sb.appendCodePoint(b); b = readByte();}
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {minus = true; b = readByte();}
        if (b < '0' || '9' < b) throw new NumberFormatException();
        for (; ; b = readByte()) {
            if ('0' <= b && b <= '9') n = n * 10 + b - '0';
            else if (b == -1 || !(33 <= b && b <= 126)) return minus ? -n : n;
            else throw new NumberFormatException();
        }
    }
    public int nextInt() {return Math.toIntExact(nextLong());}
    public double nextDouble() {return Double.parseDouble(next());}
}


@SuppressWarnings("unchecked")
class Stack<T> {
    private static final int DEFAULT_SIZE = 64;
    private T[] s;
    private int size;
    private int tail = 0;
    public Stack(final int capacity) {this.s = (T[]) new Object[capacity];this.size = capacity;}
    public Stack() {this(DEFAULT_SIZE);}
    public T peek() {
        if (tail == 0) throw new NoSuchElementException("No Elements.");
        return s[tail - 1];
    }
    public T getFromHead(int index) {
        if (index >= tail || index < 0) throw new NoSuchElementException("index out of Bounds.");
        return s[index];
    }
    public T getFromTail(int index) {
        if (index >= tail || index < 0) throw new NoSuchElementException("index out of Bounds.");
        return s[tail - 1 - index];
    }
    public void add(final T v) {
        if (tail == size) grow();
        s[tail++] = v;
    }
    public void add(final T... vals) {for (T v : vals) add(v);}
    public T poll() {
        if (tail == 0) throw new NoSuchElementException("No Elements.");
        return s[--tail];
    }
    public int size() {return tail;}
    private void grow() {
        final T[] grown = (T[]) new Object[size << 1];
        System.arraycopy(s, 0, grown, 0, tail);
        s = grown;
        size <<= 1;
    }
    public Iterator<T> iterator() {return new StackIterator();}
    private class StackIterator implements Iterator<T> {
        private int i = tail - 1;
        public boolean hasNext() {return i >= 0;}
        public T next() {return s[i--];}
    }
}