using System;
using System.Collections.Generic;
using static Assistant.Input;
using static Assistant.Debug;
using System.Linq;
using Assistant;

namespace ABC159E
{
    class Program
    {
        static void Main(string[] args)
        {
            var H = RInt;
            var W = RInt;
            var K = RInt;

            string[] map = new string[H];
            for (int i = 0; i < H; i++)
            {
                map[i] = Console.ReadLine();
            }

            //Bit全探索
            int ans = int.MaxValue;
            var bit = BitSearch(H - 1);
            foreach (var b in bit)
            {
                int cand = b.Count(x => x == 1);

                int[] count = new int[H];
                for (int i = 0; i < W; i++)
                {
                    int index = 0;
                    int[] countH = new int[H];
                    for (int j = 0; j < H; j++)
                    {
                        if (j > 0 && b[j - 1] == 1) index++;
                        if (map[j][i] == '1')
                        {
                            countH[index]++;
                            if (countH[index] > K)
                            {
                                countH[0] = -1;
                                cand = int.MaxValue;
                                break;
                            }

                            count[index]++;
                            if (count[index] > K)
                            {
                                count = new int[H];
                                i--;
                                cand++;
                                break;
                            }
                        }
                    }
                    if (countH[0] == -1) break;
                }

                ans = Math.Min(cand, ans);
            }

            Console.WriteLine(ans);
        }

        /// <summary>
        /// bit全探索　メソッドバージョン
        /// </summary>
        /// <param name="n"></param>
        /// <returns>int[1<<n][]</returns>
        public static int[][] BitSearch(int n)
        {
            int[][] ret = new int[1 << n][];
            for (int bit = 0; bit < (1 << n); ++bit)
            {
                int[] S = new int[n];
                for (int i = 0; i < n; i++)
                {
                    if ((bit & (1 << i)) != 0)
                    {
                        S[i] = 1;
                    }
                }

                ret[bit] = S.ToArray();
            }
            return ret;
        }
    }


}

namespace Assistant
{
    static class Input
    {
        static List<string> line = new List<string>();
        static int index = 0;
        static String RNext()
        {
            if (line.Count <= index) line.AddRange(Console.ReadLine().Split());
            return line[index++];
        }
        public static int RInt => int.Parse(RNext());
        public static long RLong => long.Parse(RNext());
        public static int[] RInts => Console.ReadLine().Split().Select(int.Parse).ToArray();
        public static long[] RLongs => Console.ReadLine().Split().Select(long.Parse).ToArray();
        public static string RString => RNext();
        //以下未テスト
        public static int[] RIntsC(int c) => Enumerable.Repeat(0, c).Select(x => int.Parse(RNext())).ToArray();
        public static long[] RLongsC(int c) => Enumerable.Repeat(0, c).Select(x => long.Parse(RNext())).ToArray();
        public static char[][] RMap(int h) => Enumerable.Repeat(0, h).Select(x => Console.ReadLine().ToCharArray()).ToArray();
    }

    static class Debug
    {
        static public void Draw2D<T>(T[,] map, int mode = 1)
        {
#if DEBUG
            int W = map.GetLength(0);
            int H = map.GetLength(1);

            string[,] map2 = new string[W + 1, H + 1];
            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (i == 0 && j == 0) map2[i, j] = 0.ToString();
                    else if (i == 0) map2[i, j] = (j - 1).ToString();
                    else if (j == 0) map2[i, j] = (i - 1).ToString();
                    else map2[i, j] = map[i - 1, j - 1].ToString();
                }
            }

            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (mode == 0) Console.Write(map2[i, j].Last());
                    if (mode == 1) Console.Write(map2[i, j] + " ");
                }
                Console.WriteLine();
            }
            Console.WriteLine();
#endif
        }

        public static void Draw1D<T>(T[] array, int mode = 0)
        {
#if DEBUG
            Console.WriteLine(string.Join(" ", array));
#endif
        }
    }
}


Measured memory_bytes: 17844