Problem:
Score : 1000 points

Problem Statement

Note the unusual memory limit.

For a rectangular grid where each square is painted white or black, we define its complexity as follows:

If all the squares are black or all the squares are white, the complexity is 0.

Otherwise, divide the grid into two subgrids by a line parallel to one of the sides of the grid, and let c_1 and c_2 be the complexities of the subgrids. There can be multiple ways to perform the division, and let m be the minimum value of \max(c_1, c_2) in those divisions. The complexity of the grid is m+1.

You are given a grid with H horizontal rows and W vertical columns where each square is painted white or black.
HW characters from A_{11} to A_{HW} represent the colors of the squares.
A_{ij} is # if the square at the i-th row from the top and the j-th column from the left is black, and A_{ij} is . if that square is white.

Find the complexity of the given grid.

Constraints

1 \leq H,W \leq 185

A_{ij} is # or ..

Input

Input is given from Standard Input in the following format:

H W
A_{11}A_{12}...A_{1W}
:
A_{H1}A_{H2}...A_{HW}

Output

Print the complexity of the given grid.

Sample Input 1

3 3
...
.##
.##

Sample Output 1

2

Let us divide the grid by the boundary line between the first and second columns.
The subgrid consisting of the first column has the complexity of 0, and the subgrid consisting of the second and third columns has the complexity of 1, so the whole grid has the complexity of at most 2.

Sample Input 2

6 7
.####.#
#....#.
#....#.
#....#.
.####.#
#....##

Sample Output 2

4

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.io.*;
import java.nio.CharBuffer;
import java.util.NoSuchElementException;

public class Main {

    public static void main(String[] args) {
        SimpleScanner scanner = new SimpleScanner(System.in);
        PrintWriter writer = new PrintWriter(System.out);

        int r = scanner.nextInt();
        int c = scanner.nextInt();
        char[][] grid = new char[r][];
        int[][] bit = new int[r + 1][c + 1];
        for (int i = 0; i < r; ++i) {
            grid[i] = scanner.next().toCharArray();
            for (int j = 0; j < c; ++j) {
                if (grid[i][j] == '#') {
                    bitAdd(bit, r, c, i + 1, j + 1, 1);
                }
            }
        }
        writer.println(solve(bit, 1, 1, r, c));

        writer.close();
    }

    private static int solve(int[][] bit, int x0, int y0, int x1, int y1) {
        int sum = bitSum(bit, x1, y1) - bitSum(bit, x0 - 1, y1) - bitSum(bit, x1, y0 - 1) + bitSum(bit, x0 - 1, y0 - 1);
        if (sum == 0 || sum == (x1 - x0 + 1) * (y1 - y0 + 1))
            return 0;
        int ans = Integer.MAX_VALUE;
        for (int i = x0; i < x1; ++i) {
            int k1 = solve(bit, x0, y0, i, y1);
            int k2 = solve(bit, i + 1, y0, x1, y1);
            int k = Math.max(k1, k2) + 1;
            ans = Math.min(ans, k);
        }
        for (int i = y0; i < y1; ++i) {
            int k1 = solve(bit, x0, y0, x1, i);
            int k2 = solve(bit, x0, i + 1, x1, y1);
            int k = Math.max(k1, k2) + 1;
            ans = Math.min(ans, k);
        }
        return ans;
    }

    private static int bitSum(int[][] bit, int x, int y) {
        int sum = 0;
        for (int i = x; i > 0; i -= (i & -i)) {
            for (int j = y; j > 0; j -= (j & -j)) {
                sum += bit[i][j];
            }
        }
        return sum;
    }


    private static void bitAdd(int[][] bit, int r, int c, int x, int y, int val) {
        for (int i = x; i <= r; i += (i & -i)) {
            for (int j = y; j <= c; j += (j & -j)) {
                bit[i][j] += val;
            }
        }
    }

    private static class SimpleScanner {

        private static final int BUFFER_SIZE = 10240;

        private Readable in;
        private CharBuffer buffer;
        private boolean eof;

        SimpleScanner(InputStream in) {
            this.in = new BufferedReader(new InputStreamReader(in));
            buffer = CharBuffer.allocate(BUFFER_SIZE);
            buffer.limit(0);
            eof = false;
        }


        private char read() {
            if (!buffer.hasRemaining()) {
                buffer.clear();
                int n;
                try {
                    n = in.read(buffer);
                } catch (IOException e) {
                    n = -1;
                }
                if (n <= 0) {
                    eof = true;
                    return '\0';
                }
                buffer.flip();
            }
            return buffer.get();
        }

        void checkEof() {
            if (eof)
                throw new NoSuchElementException();
        }

        char nextChar() {
            checkEof();
            char b = read();
            checkEof();
            return b;
        }

        String next() {
            char b;
            do {
                b = read();
                checkEof();
            } while (Character.isWhitespace(b));
            StringBuilder sb = new StringBuilder();
            do {
                sb.append(b);
                b = read();
            } while (!eof && !Character.isWhitespace(b));
            return sb.toString();
        }

        int nextInt() {
            return Integer.valueOf(next());
        }

        long nextLong() {
            return Long.valueOf(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }
    }
}