using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using static System.Console;
using static System.Math;
using static MyIO;
using static MyUtil;
using static BinaryRepresentation;

public class E
{
	public static void Main() => (new Solver()).Solve();
}

public class Solver
{
	public void Solve()
	{
		int H = GetInt();
		int W = GetInt();
		int K = GetInt();
		var grid = new int[H, W];
		for(int i = 0; i < H; i++)
		{
			var S = GetString();
			for(int j = 0; j < W; j++)
				grid[i, j] = (int)(S[j] - '0');
		}

		int ans = int.MaxValue;
		for(int s = 0; s < (1 << (H - 1)); s++)
		{
			int op = popcnt(s);
			var grp = new int[H];  // grp[0] = 0;
			int p = 0;
			for(int i = 0; i < H - 1; i++)
			{
				if(contains(s, i))
				{
					grp[i + 1] = ++p;
				}
				else
				{
					grp[i + 1] = p;
				}
			}

			bool valid = true;
			var cnt = new int[p + 1];
			for(int i = 0; i < W; i++)
			{
				var add = new int[p + 1];
				for(int j = 0; j < H; j++)
				{
					if(grid[j, i] == 1)
						add[grp[j]]++;
				}

				if(Enumerable.Range(0, p + 1).Any(x => add[x] > K))
				{
					valid = false;
					break;
				}
				if(Enumerable.Range(0, p + 1).Any(x => cnt[x] + add[x] > K))
				{
					cnt = add;
					op++;
				}
				else
				{
					for(int j = 0; j <= p; j++)
						cnt[j] += add[j];				
				}
			}
			if(valid)
				chmin(ref ans, op);
		}

		WriteLine(ans);
	}
	
	private bool contains(int s, int x) => ((s >> x) & 1) == 1;
}

public static class BinaryRepresentation
{
	/*
	* initialize: array = { 0, 1, 2, ..., N - 1 } 
	* do { ... } while(nextPermutaion(ref array));
	*/
	private static bool nextPermutation(ref int[] array)
	{
		int len = array.Length;
		int i = len - 2;
		while((i >= 0) && (array[i] > array[i + 1]))
			i--;
		if(i < 0)
			return false;

		int pivot = array[i];
		int j = i + 1;
		while((j < len - 1) && (array[j + 1] > pivot))
			j++;
		
		int tmp = array[i];
		array[i] = array[j];
		array[j] = tmp;
		
		Array.Reverse(array, i + 1, len - i - 1);

		return true;
	}

	/* 
	* nCk
	* initialize: comb = (1 << k) - 1
	* while(comb < (1 << n)) { ...; comb = nextCombination(comb); };
	*/
	public static int nextCombination(int comb)
	{
		int x = comb & -comb;
		int y = comb + x;
		return (((comb & ~y) /x) >> 1) | y;
	}

	public static int popcnt(int v) {
		v = v - ((v >> 1) & 0x55555555);
		v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
		return ((v + (v >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
	}

	public static long popcnt(long v) {
		v = v - ((v >> 1) & 0x5555555555555555);
		v = (v & 0x3333333333333333) + ((v >> 2) & 0x3333333333333333);
		return ((v + (v >> 4) & 0xF0F0F0F0F0F0F0F) * 0x101010101010101) >> 56;
	}
}

public static class MyIO
{
	private static string[] args = null;
	private static int num = -1;
	private static int used = -1;

	private static string getArg()
	{
		if(used == num)
		{
			args = Console.ReadLine().Split(' ');
			num = args.Length;
			used = 0;
		}
		return args[used++];
	}

	public static int GetInt() => int.Parse(getArg());
	public static long GetLong() => long.Parse(getArg());
	public static double GetDouble() => double.Parse(getArg());
	public static decimal GetDecimal() => decimal.Parse(getArg());
	public static string GetString() => getArg();
	public static char GetChar() => getArg()[0];
	public static int[] GetInts(long N) => Enumerable.Range(0, (int)N).Select(_ => GetInt()).ToArray();
	public static long[] GetLongs(long N) => Enumerable.Range(0, (int)N).Select(_ => GetLong()).ToArray();
	public static double[] GetDoubles(long N) => Enumerable.Range(0, (int)N).Select(_ => GetDouble()).ToArray();
	public static decimal[] GetDecimals(long N) => Enumerable.Range(0, (int)N).Select(_ => GetDecimal()).ToArray();
	public static string[] GetStrings(long N) => Enumerable.Range(0, (int)N).Select(_ => GetString()).ToArray();
}

public static class MyUtil
{
	public static void chmax<T>(ref T x, T y) where T : IComparable<T>
	{
		if(x.CompareTo(y) < 0)
			x = y;
	}

	public static void chmin<T>(ref T x, T y) where T : IComparable<T>
	{
		if(x.CompareTo(y) > 0)
			x = y;
	}

	public static void swap<T>(ref T x, ref T y)
	{
		T tmp = x;
		x = y;
		y = tmp;
	}
}

Measured memory_bytes: 17324