using System;
using System.Linq;
using System.Collections.Generic;
using System.Diagnostics;
using Debug = System.Diagnostics.Trace;
using SB = System.Text.StringBuilder;
using static System.Math;
using Number = System.Int32;
using System.Numerics;
namespace Program {
	using N = Int32;
	public class Solver {
		Random rnd = new Random(0);
		public void Solve() {
			var n = ri;
			var m = ri;
			var a = Enumerate(n, x => rs);
			//(i,j),(k,j) から complexity を保って伸ばせる右側の max
			var R = Enumerate(n, x => Enumerate(m, y => new int[n]));

			//(i,j),(i,l) から complexity を保って伸ばせる下側の max
			var D = Enumerate(n, x => Enumerate(m, y => new int[m]));
			for (N i = 0; i < n; i++)
				for (N j = 0; j < m; j++) {
					R[i][j][i] = j;
					while (R[i][j][i] + 1 < m && a[i][R[i][j][i] + 1] == a[i][j]) R[i][j][i]++;
					D[i][j][j] = i;
					while (D[i][j][j] + 1 < n && a[D[i][j][j] + 1][j] == a[i][j]) D[i][j][j]++;
				}
			for (int i = 0; i < n; i++)
				for (int j = 0; j < m; j++) {
					for (int k = i + 1; k < n; k++)
						if (a[i][j] == a[k][j]) R[i][j][k] = Min(R[k][j][k], R[i][j][k - 1]);
						else R[i][j][k] = (N)(j - 1);
					for (int l = j + 1; l < m; l++)
						if (a[i][j] == a[i][l]) D[i][j][l] = Min(D[i][l][l], D[i][j][l - 1]);
						else D[i][j][l] = (N)(i - 1);
				}

			var NR = Enumerate(n, x => Enumerate(m, y => new int[n]));
			var ND = Enumerate(n, x => Enumerate(m, y => new int[m]));
			for (int rank = 0; ; rank++) {
				if (R[0][0][n - 1] == m - 1) { Console.WriteLine(rank); return; }
				for (int i = 0; i < n; i++)
					for (int j = 0; j < m; j++) {
						for (int k = i; k < n; k++) {
							var nj = R[i][j][k];
							if (nj + 1 < m) nj = R[i][nj + 1][k];
							NR[i][j][k] = nj;
						}
						for (int l = j; l < m; l++) {
							var ni = D[i][j][l];
							if (ni + 1 < n) ni = D[ni + 1][j][l];
							ND[i][j][l] = ni;
						}
					}
				for (int i = 0; i < n; i++)
					for (int j = 0; j < m; j++) {
						for (int k = 0; k < n; k++) {
							var nj = NR[i][j][k];
							while (nj + 1 < m && ND[i][j][nj + 1] >= k) nj++;
							R[i][j][k] = nj;
						}
						for (int l = 0; l < m; l++) {
							var ni = ND[i][j][l];
							while (ni + 1 < n && NR[i][j][ni + 1] >= l) ni++;
							D[i][j][l] = ni;
						}
					}
			}


		}
		const long INF = 1L << 60;
		static int[] dx = { -1, 0, 1, 0 };
		static int[] dy = { 0, 1, 0, -1 };
		int ri { get { return sc.Integer(); } }
		long rl { get { return sc.Long(); } }
		double rd { get { return sc.Double(); } }
		string rs { get { return sc.Scan(); } }
		public IO.StreamScanner sc = new IO.StreamScanner(Console.OpenStandardInput());

		static T[] Enumerate<T>(int n, Func<int, T> f) {
			var a = new T[n];
			for (int i = 0; i < a.Length; ++i) a[i] = f(i);
			return a;
		}
		static public void Swap<T>(ref T a, ref T b) { var tmp = a; a = b; b = tmp; }
	}
}

#region main
static class Ex {
	static public string AsString(this IEnumerable<char> ie) { return new string(ie.ToArray()); }
	static public string AsJoinedString<T>(this IEnumerable<T> ie, string st = " ") {
		return string.Join(st, ie);
	}
	static public void Main() {
		Console.SetOut(new Program.IO.Printer(Console.OpenStandardOutput()) { AutoFlush = true });
		var solver = new Program.Solver();
		//* 
		var t = new System.Threading.Thread(solver.Solve, 50000000);
		//t.Start();
		//t.Join();
		//*/
		solver.Solve();
		Console.Out.Flush();
	}
}
#endregion
#region Ex
namespace Program.IO {
	using System.IO;
	using System.Text;
	using System.Globalization;

	public class Printer : StreamWriter {
		public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
		public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { }
	}

	public class StreamScanner {
		public StreamScanner(Stream stream) { str = stream; }

		public readonly Stream str;
		private readonly byte[] buf = new byte[1024];
		private int len, ptr;
		public bool isEof = false;
		public bool IsEndOfStream { get { return isEof; } }

		private byte read() {
			if (isEof) return 0;
			if (ptr >= len) {
				ptr = 0;
				if ((len = str.Read(buf, 0, 1024)) <= 0) {
					isEof = true;
					return 0;
				}
			}
			return buf[ptr++];
		}

		public char Char() {
			byte b = 0;
			do b = read(); while ((b < 33 || 126 < b) && !isEof);
			return (char)b;
		}
		public string Scan() {
			var sb = new StringBuilder();
			for (var b = Char(); b >= 33 && b <= 126; b = (char)read()) sb.Append(b);
			return sb.ToString();
		}
		public string ScanLine() {
			var sb = new StringBuilder();
			for (var b = Char(); b != '\n' && b != 0; b = (char)read()) if (b != '\r') sb.Append(b);
			return sb.ToString();
		}
		public long Long() { return isEof ? long.MinValue : long.Parse(Scan()); }
		public int Integer() { return isEof ? int.MinValue : int.Parse(Scan()); }
		public double Double() { return isEof ? double.NaN : double.Parse(Scan(), CultureInfo.InvariantCulture); }
	}
}

#endregion


Measured memory_bytes: 133496