using AtCoderBeginnerContest159.Questions;
using AtCoderBeginnerContest159.Extensions;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace AtCoderBeginnerContest159.Questions
{
    public class QuestionE : AtCoderQuestionBase
    {
        int h, w, k;
        bool[][] isWhite;

        // 復習
        public override IEnumerable<object> Solve(TextReader inputStream)
        {
            var hwk = inputStream.ReadIntArray();
            h = hwk[0];
            w = hwk[1];
            k = hwk[2];

            isWhite = new bool[h][];
            for (int i = 0; i < h; i++)
            {
                isWhite[i] = inputStream.ReadLine().Select(c => c == '1').ToArray();
            }

            int min = int.MaxValue;
            for (int bitFlag = 0; bitFlag < 1 << (h - 1); bitFlag++)
            {
                var result = SolveFor(bitFlag);
                min = Math.Min(min, result);
            }

            yield return min;
        }

        private int SolveFor(int bitFlag)
        {
            checked
            {
                var rowGroups = new List<List<int>>();
                var group = new List<int>();
                rowGroups.Add(group);
                group.Add(0);

                for (int i = 1; i < h; i++)
                {
                    if ((bitFlag & 0x01 << (i - 1)) != 0)
                    {
                        group = new List<int>();
                        rowGroups.Add(group);
                    }

                    group.Add(i);
                }

                var splitCount = rowGroups.Count - 1;
                var whiteCounts = new int[rowGroups.Count];

                for (int i = 0; i < w; i++)
                {
                    var currentWhiteCounts = rowGroups.Select(g => g.Count(r => isWhite[r][i])).ToArray();

                    if (currentWhiteCounts.Any(n => n > k))
                    {
                        // この割り方だとNGであることを返す
                        return int.MaxValue;
                    }

                    for (int j = 0; j < whiteCounts.Length; j++)
                    {
                        whiteCounts[j] += currentWhiteCounts[j];
                    }

                    if (whiteCounts.Any(n => n > k))
                    {
                        // 超えてたら手前で割ってリセット
                        whiteCounts = currentWhiteCounts;
                        splitCount++;
                    }
                }

                return splitCount;
            }
        }
    }
}


namespace AtCoderBeginnerContest159
{
    class Program
    {
        static void Main(string[] args)
        {
            IAtCoderQuestion question = new QuestionE();    // 問題に合わせて書き換え
            var answers = question.Solve(Console.In);
            foreach (var answer in answers)
            {
                Console.WriteLine(answer);
            }
        }
    }
}

#region Base Classes

namespace AtCoderBeginnerContest159.Questions
{

    public interface IAtCoderQuestion
    {
        IEnumerable<object> Solve(string input);
        IEnumerable<object> Solve(TextReader inputStream);
    }

    public abstract class AtCoderQuestionBase : IAtCoderQuestion
    {
        public IEnumerable<object> Solve(string input)
        {
            var stream = new MemoryStream(Encoding.Unicode.GetBytes(input));
            var reader = new StreamReader(stream, Encoding.Unicode);

            return Solve(reader);
        }

        public abstract IEnumerable<object> Solve(TextReader inputStream);
    }

}

#endregion

#region Extensions

namespace AtCoderBeginnerContest159.Extensions
{
    internal static class TextReaderExtensions
    {
        internal static int ReadInt(this TextReader reader) => int.Parse(ReadString(reader));
        internal static long ReadLong(this TextReader reader) => long.Parse(ReadString(reader));
        internal static double ReadDouble(this TextReader reader) => double.Parse(ReadString(reader));
        internal static string ReadString(this TextReader reader) => reader.ReadLine();

        internal static int[] ReadIntArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(int.Parse).ToArray();
        internal static long[] ReadLongArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(long.Parse).ToArray();
        internal static double[] ReadDoubleArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(double.Parse).ToArray();
        internal static string[] ReadStringArray(this TextReader reader, char separator = ' ') => reader.ReadLine().Split(separator);
    }
}

#endregion

Measured memory_bytes: 17740