using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using static System.Console;
using static System.Math;

class program
{
    static public int mod = 1000000007;
    static void Main()
    {
        var z = inta();
        int h = z[0], w = z[1], k = z[2];
        var s = new string[h];
        var bitct = new int[h][];
        int itr = 1 << (h - 1);
        for (int i = 0; i < h; i++)
        {
            s[i] = ReadLine();
            bitct[i] = new int[w + 1];
            for (int j = 0; j < w; j++)
            {
                bitct[i][j + 1] = bitct[i][j];
                if (s[i][j] == '1')
                    bitct[i][j + 1]++;
            }
        }
        int ans = int.MaxValue;
        for (int i = 0; i < itr; i++)
        {
            var bdct = new int[h][];
            int ct = 0;
            bdct[0] = new int[w + 1];
            int div = 0;
            int divw = 0;
            bool isans = false;
            for (int j = 0; j < h; j++)
            {
                for (int a = 1; a <= w; a++)
                {
                    bdct[ct][a] += bitct[j][a];
                }
                if ((i & (1 << j)) != 0)
                {
                    ct++;
                    div++;
                    bdct[ct] = new int[w + 1];
                }
            }

            for (int a = 1; a <= w; a++)
            {
                for (int j = 0; j < ct + 1; j++)
                {
                    if ((bdct[j][a] - bdct[j][divw]) > k)
                    {
                        div++;
                        divw = a - 1;
                        break;
                    }
                    else if(j == ct)
                    {
                        isans = true;
                    }
                }
            }

            if (ans > div && isans)
                ans = div;
        }
        Write(ans);
    }

    static int pow(int a, int b) { string binary = Convert.ToString(b, 2); int bileng = binary.Length; long a_power = a; long value = 1; for (int i = bileng - 1; i >= 0; i--) { if (binary[i] == '1') { value = value * a_power % mod; } a_power = a_power * a_power % mod; } return (int)value; }
    static string rstr() { return ReadLine(); }
    static int rint() { return int.Parse(ReadLine()); }
    static long rlong() { return long.Parse(ReadLine()); }
    static string[] stra() { return ReadLine().Split(' '); }
    static char[] chara() { string[] a = stra(); string b = ""; for (int i = 0; i < a.Length; i++) { b += a[i]; } return b.ToCharArray(); }
    static int[] inta() { string[] read_str_array = ReadLine().Split(' '); int[] return_int_array = new int[read_str_array.Length]; for (int countup_i = 0; countup_i < read_str_array.Length; countup_i++) { return_int_array[countup_i] = int.Parse(read_str_array[countup_i]); } return return_int_array; }
    static long[] longa() { string[] read_str_array = ReadLine().Split(' '); long[] return_long_array = new long[read_str_array.Length]; for (long countup_i = 0; countup_i < read_str_array.Length; countup_i++) { return_long_array[countup_i] = long.Parse(read_str_array[countup_i]); } return return_long_array; }
    static double[] doublea() { string[] read_str_array = ReadLine().Split(' '); double[] return_double_array = new double[read_str_array.Length]; for (long countup_i = 0; countup_i < read_str_array.Length; countup_i++) { return_double_array[countup_i] = double.Parse(read_str_array[countup_i]); } return return_double_array; }
    static long GCD(long a, long b) { if (a < b) { long temp = a; a = b; b = temp; } if (a % b == 0) { return b; } else { long temp = b; b = a % b; a = temp; return GCD(a, b); } }
    static long LCM(long a, long b) { return a * b / GCD(a, b); }
    static void WriteArray2(int[,] a, int b, int c) { for (int i = 0; i < b; i++) { for (int j = 0; j < c; j++) { if (j != 0) Write(" "); Write(a[i, j]); } WriteLine(); } }
    static void WriteArray(int[] a, int b) { for (int i = 0; i < b; i++) { Write(" "); Write(a[i]); } WriteLine(); }
}


Measured memory_bytes: 38484