#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <cstdio>
#include <cmath>
#include <tuple>
#include <iomanip>
#include <numeric>
#include <unordered_map>
#include <sstream>   
#include<limits.h>
#include<float.h>
#include<list>
#include <array>
#include <complex>
#include<stdio.h>
#include<string.h>
#include <bitset>

using namespace std;
#define int  long long
 
#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807LL
#define I64_MAX2 1223372036854775807LL
#define INF I64_MAX2
#define MOD 1000000007
#define MEM_SIZE 10000
#define DEBUG_OUT true
#define ALL(x) (x).begin(),(x).end()

// int DP[MEM_SIZE][MEM_SIZE] = {0};
// int GMEM[MEM_SIZE][MEM_SIZE] = {0};
template<typename T> void DEBUG(T e){if(DEBUG_OUT == false)return; std::cout << e <<" ";}
template<typename T> void DEBUG(const std::vector<T>& v){if(DEBUG_OUT == false)return;for(const auto& e : v){std::cout<< e << " "; } std::cout << std::endl;}
template<typename T> void DEBUG(const std::vector<std::vector<T> >& vv){if(DEBUG_OUT == false)return;for(const auto& v : vv){ DEBUG(v); } }
template<class T,class... Ts> void DEBUG(T d, Ts... e){if(DEBUG_OUT == false)return;DEBUG(d);DEBUG(e...);};
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template< typename T1, typename T2 > inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template< typename T1, typename T2 > inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }
template<typename T> vector<T> make_v(size_t a){return vector<T>(a);}
template<typename T,typename... Ts> auto make_v(size_t a,Ts... ts){   return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...)); }
template<typename T,typename V> typename enable_if<is_class<T>::value==0>::type fill_v(T &t,const V &v){t=v;}
template<typename T,typename V> typename enable_if<is_class<T>::value!=0>::type fill_v(T &t,const V &v){ for(auto &e:t) fill_v(e,v); }
template<typename T,typename U,typename... V> typename enable_if<is_same<T, U>::value!=0>::type fill_v(U &u,const V... v){u=U(v...);}
template<typename T,typename U,typename... V> typename enable_if<is_same<T, U>::value==0>::type fill_v(U &u,const V... v){ for(auto &e:u) fill_v<T>(e,v...);}
//FUNCTION

//BELOW
void solve(void)
{
  int H, W, K;
  cin >> H >> W >> K;

  auto data = make_v<int>(H, W);
  for (int i = 0; i < H; i++)
  {
    for (int j = 0; j < W; j++)
    {
      char c;
      cin >> c;
      data[i][j] = c - '0';
    }
  }
  int ANS = INF;

  for (int b = 0; b < (1<<H); b++)
  {
    vector<int> cut(H - 1, 0);
    int check = 0;
    int CUT_NUM = __builtin_popcount(b);
    for (int i = 0; i < H-1; i++)
    {
      if ((b >> i) & 1)
        cut[i] = 1;
    }
    // DEBUG(cut);
    // DEBUG(b);
    auto H_CUT = make_v<int>(CUT_NUM + 1, W);
    fill_v(H_CUT, 0);
    for (int w = 0; w < W; w++)
    {
      int index = 0;
      for (int h = 0; h < H; h++)
      {
        H_CUT[index][w] += data[h][w];
        if (h != H - 1)
        {
          if (cut[h] == 1)
            index++;
        }
      }
    }

    for (int w = 0; w < W; w++)
    {
      for (int h = 0; h < CUT_NUM+1; h++)
      {
        if(H_CUT[h][w] > K) check = INF;
      }
      
    }
    
    // DEBUG(H_CUT);

    vector<int> sum(CUT_NUM + 1, 0);
    int W_CUT = 0;
    for (int w = 0; w < W; w++)
    {
      for (int h = 0; h < CUT_NUM + 1; h++)
      {
        sum[h] += H_CUT[h][w];
        int X = sum[h];
        if (X > K)
        {
          W_CUT++;
          for (int i = 0; i < CUT_NUM + 1; i++)
          {
            sum[i] = H_CUT[i][w];
          }
          break;
        }
      }
    }
    // DEBUG(CUT_NUM,W_CUT,check,"\n");
    chmin(ANS, CUT_NUM + W_CUT + check);
  }
  cout << ANS << endl;

  return;
}

int32_t main(int32_t argc, const char *argv[])
{
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(9);
  solve();

  return 0;
}


Measured memory_bytes: 384