using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        //var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        //Console.SetOut(sw);

        var H = sc.ReadInt();
        var W = sc.ReadInt();
        var K = sc.ReadInt();
        var S = sc.ReadStrArray(H);

        var sum = new int[H + 1, W + 1];
        for (int i = 0; i < H; i++)
        {
            for (int j = 0; j < W; j++)
            {
                sum[i + 1, j + 1] = sum[i + 1, j] + sum[i, j + 1] - sum[i, j] + (S[i][j] == '1' ? 1 : 0);
            }
        }
        if (sum[H, W] <= K)
        {
            Console.WriteLine(0);
            return;
        }

        var ans = int.MaxValue;
        for (int x = 0; x < Math.Pow(2, H - 1); x++)
        {
            if (x == 2) { }
            var cutH = new List<int>() { 0 };
            for (int y = 0; y < H - 1; y++)
            {
                var z = (x >> y) & 1;
                if (z == 1) cutH.Add(y + 1);
            }
            cutH.Add(H);

            var cutHCount = cutH.Count - 2;
            var cutWCount = 0;
            var prevW = 0;
            var counter = new int[cutH.Count];
            var ok = false;
            for (int w = 0; w < W; w++)
            {
                var over = false;
                for (int i = 1; i < cutH.Count; i++)
                {
                    counter[i] = sum[cutH[i], w + 1]
                        - sum[cutH[i - 1], w + 1]
                        - sum[cutH[i], prevW]
                        + sum[cutH[i - 1], prevW];
                    if (counter[i] > K) over = true;
                }
                if (over)
                {
                    over = false;
                    prevW = w;
                    for (int i = 1; i < cutH.Count; i++)
                    {
                        counter[i] = sum[cutH[i], w + 1]
                            - sum[cutH[i - 1], w + 1]
                            - sum[cutH[i], prevW]
                            + sum[cutH[i - 1], prevW];
                        if (counter[i] > K) over = true;
                    }
                    if (over) continue;
                    cutWCount++;
                }
                if (w == W - 1) ok = true;
            }
            if (cutHCount + cutWCount == 3) { }
            if (ok) ans = Math.Min(ans, cutHCount + cutWCount);
        }
        Console.WriteLine(ans);

        //Console.Out.Flush();
    }
    static Scanner sc = new Scanner();
}

class Scanner
{
    string[] S = new string[0];
    int Index = 0;
    char[] Separators = new char[] { ' ' };

    public string Next()
    {
        if (this.Index < this.S.Length) return this.S[this.Index++];
        var line = "";
        while (line == "") line = Console.ReadLine();
        this.S = line.Split(this.Separators, StringSplitOptions.RemoveEmptyEntries);
        if (this.S.Length == 0) return this.Next();
        this.Index = 0;
        return this.S[this.Index++];
    }
    public string ReadStr()
    {
        return this.Next();
    }
    public char ReadChar()
    {
        return this.Next()[0];
    }
    public int ReadInt()
    {
        return int.Parse(this.Next());
    }
    public uint ReadUInt()
    {
        return uint.Parse(this.Next());
    }
    public long ReadLong()
    {
        return long.Parse(this.Next());
    }
    public double ReadDouble()
    {
        return double.Parse(this.Next());
    }
    public Tuple<int, int> ReadTup(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<long, long> ReadTupLong(int add = 0)
    {
        return Tuple.Create(this.ReadLong() + add, this.ReadLong() + add);
    }
    public Tuple<int, int, int> ReadTup3(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public Tuple<int, int, int, int> ReadTup4(int add = 0)
    {
        return Tuple.Create(this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add, this.ReadInt() + add);
    }
    public int[] ReadIntArray(int n)
    {
        var array = new int[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadInt();
        }
        return array;
    }
    public long[] ReadLongArray(int n)
    {
        var array = new long[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadLong();
        }
        return array;
    }
    public double[] ReadDoubleArray(int n)
    {
        var array = new double[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadDouble();
        }
        return array;
    }
    public char[] ReadCharArray(int n)
    {
        var array = new char[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadChar();
        }
        return array;
    }
    public string[] ReadStrArray(int n)
    {
        var array = new string[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadStr();
        }
        return array;
    }
    public Tuple<long, long>[] ReadTupLongArray(int n, int add = 0)
    {
        var array = new Tuple<long, long>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTupLong(add);
        }
        return array;
    }
    public Tuple<int, int>[] ReadTupArray(int n, int add = 0)
    {
        var array = new Tuple<int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup(add);
        }
        return array;
    }
    public Tuple<int, int, int>[] ReadTup3Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup3(add);
        }
        return array;
    }
    public Tuple<int, int, int, int>[] ReadTup4Array(int n, int add = 0)
    {
        var array = new Tuple<int, int, int, int>[n];
        for (int i = 0; i < n; i++)
        {
            array[i] = this.ReadTup4(add);
        }
        return array;
    }
}


Measured memory_bytes: 11296