using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;

namespace AtCorder
{
	public class Program
	{
		static void Main(string[] args)
		{
			var cin = new Scanner2();
			int h = cin.Int();
			int w = cin.Int();
			int k = cin.Int();

			var s = new string[h];
			for (int i = 0; i < h; i++) {
				s[i] = cin.Next();
			}

			
			int hh = h - 1;
			int ww = w - 1;
			long ans = h * w;
			for (int i = 0; i < 1 << hh; i++) {
				var hs = new List<int>();
				hs.Add(0);
				for (int j = 0; j < hh; j++) {
					if (((1 << j) & i) != 0) {
						hs.Add(j + 1);
					}
				}

				hs.Add(h);
				var cnt = new int[hs.Count - 1];
				long temp = hs.Count - 2;
				int width = 1;
				bool okok = true;
				for (int l = 0; l < w; l++) {
					for (int j = 0; j < hs.Count - 1; j++) {
						int st = hs[j];
						int en = hs[j + 1];
						for (int kk = st; kk < en; kk++) {
							if (s[kk][l] == '1') {
								cnt[j]++;
							}
						}
					}

					bool ok = true;
					for (int j = 0; j < hs.Count - 1; j++) {
						if (cnt[j] > k) {
							ok = false;
							break;
						}
					}

					if (ok == false) {
						if (width == 1) {
							okok = false;
							break;
						} else {
							l--;
							width = 1;
							++temp;
							for (int j = 0; j < hs.Count - 1; j++) {
								cnt[j] = 0;
							}
						}
					} else {
						++width;
					}
				}

				if (okok) {
					ans = Math.Min(ans, temp);
				}
				/*
				temp = hs.Count - 2;
				width = 1;
				for (int l = w - 1; l >= 0; l--) {
					for (int j = 0; j < hs.Count - 1; j++) {
						int st = hs[j];
						int en = hs[j + 1];
						for (int kk = st; kk < en; kk++) {
							if (s[kk][l] == '1') {
								cnt[j]++;
							}
						}
					}

					bool ok = true;
					for (int j = 0; j < hs.Count - 1; j++) {
						if (cnt[j] > k) {
							ok = false;
							break;
						}
					}

					if (ok == false) {
						if (width == 1) {
							break;
						} else {
							l++;
							width = 1;
							++temp;
							for (int j = 0; j < hs.Count - 1; j++) {
								cnt[j] = 0;
							}
						}
					} else {
						++width;
					}
				}

				ans = Math.Min(ans, temp);
				*/
			}

			Console.WriteLine(ans);
		}

		public static int PopCount(ulong bits)
		{
			bits = ((bits & 0xaaaaaaaaaaaaaaaa) >> 1) + (bits & 0x5555555555555555);
			bits = ((bits & 0xcccccccccccccccc) >> 2) + (bits & 0x3333333333333333);
			bits = ((bits & 0xf0f0f0f0f0f0f0f0) >> 4) + (bits & 0x0f0f0f0f0f0f0f0f);
			bits = ((bits & 0xff00ff00ff00ff00) >> 8) + (bits & 0x00ff00ff00ff00ff);
			bits = ((bits & 0xffff0000ffff0000) >> 16) + (bits & 0x0000ffff0000ffff);
			bits = ((bits & 0xffffffff00000000) >> 32) + (bits & 0x00000000ffffffff);
			return (int)bits;
		}
	}

	public static class Helper
	{
		public static void UpdateMin32<T>(T array, int index, int value) where T : IList<int>
			=> array[index] = Math.Min(array[index], value);
		public static void UpdateMax32<T>(T array, int index, int value) where T : IList<int>
			=> array[index] = Math.Max(array[index], value);
		public static void UpdateMin64<T>(T array, int index, long value) where T : IList<long>
			=> array[index] = Math.Min(array[index], value);
		public static void UpdateMax64<T>(T array, int index, long value) where T : IList<long>
			=> array[index] = Math.Max(array[index], value);

		public static T[] Array<T>(int n, Func<int, T> init)
			=> Enumerable.Range(0, n).Select(x => init(x)).ToArray();
		public static List<T> List<T>(int n, Func<int, T> init)
			=> Enumerable.Range(0, n).Select(x => init(x)).ToList();
	}

	public class Scanner2
	{
		private readonly char[] delimiter_ = new char[] { ' ' };
		private readonly string filePath_;
		private readonly Func<string> reader_;
		private string[] buf_;
		private int index_;

		public Scanner2(string file = "")
		{
			if (string.IsNullOrWhiteSpace(file)) {
				reader_ = Console.ReadLine;
			} else {
				filePath_ = file;
				var fs = new StreamReader(file);
				reader_ = fs.ReadLine;
			}
			buf_ = new string[0];
			index_ = 0;
		}

		public string Next()
		{
			if (index_ < buf_.Length) {
				return buf_[index_++];
			}

			string st = reader_();
			while (st == "") {
				st = reader_();
			}

			buf_ = st.Split(delimiter_, StringSplitOptions.RemoveEmptyEntries);
			if (buf_.Length == 0) {
				return Next();
			}

			index_ = 0;
			return buf_[index_++];
		}

		public int Int() => int.Parse(Next());
		public long Long() => long.Parse(Next());
		public double Double() => double.Parse(Next());

		public int[] ArrayInt(int N, int add = 0)
		{
			int[] Array = new int[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Int() + add;
			}
			return Array;
		}

		public long[] ArrayLong(int N, long add = 0)
		{
			long[] Array = new long[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Long() + add;
			}
			return Array;
		}

		public double[] ArrayDouble(int N, double add = 0)
		{
			double[] Array = new double[N];
			for (int i = 0; i < N; i++) {
				Array[i] = Double() + add;
			}
			return Array;
		}

		public void Save(string text)
		{
			if (string.IsNullOrWhiteSpace(filePath_)) {
				return;
			}

			File.WriteAllText(filePath_ + "_output.txt", text);
		}
	}
}

Measured memory_bytes: 13268