#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<LL, LL> PII;

// chmax, chmin
template <class T>
inline bool chmax(T &a, T b)
{
  if (a < b)
  {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T &a, T b)
{
  if (a > b)
  {
    a = b;
    return 1;
  }
  return 0;
}

template <typename T>
function<T(T, T)> op_max = [](T a, T b) -> T { return max(a, b); };
template <typename T>
function<T(T, T)> op_min = [](T a, T b) -> T { return min(a, b); };

template <typename T>
function<T(T, T)> op_sum = [](T a, T b) -> T { return a + b; };

namespace detail
{
  template <typename T, std::size_t NDIMS>
  struct vector_builder
  {
    using type = std::vector<typename vector_builder<T, NDIMS - 1>::type>;

    static type make(std::vector<std::size_t> dims, const T &v = {})
    {
      const auto vec = std::vector<T>(dims.empty() ? 0 : dims.back(), v);
      if (!dims.empty())
        dims.pop_back();
      return vector_builder<std::vector<T>, NDIMS - 1>::make(dims, vec);
    }
  };

  template <typename T>
  struct vector_builder<T, 1>
  {
    using type = std::vector<T>;

    static type make(std::vector<std::size_t> dims, const T &v = {})
    {
      return type(dims.empty() ? 0 : dims.back(), v);
    }
  };
} // namespace detail

template <typename T, typename... SIZE_T>
auto exvector(const T &v, SIZE_T... dims)
{
  static_assert(sizeof...(dims) != 0, "invalid dimension");
  std::vector<std::size_t> vec_dims{dims...};
  return detail::vector_builder<T, sizeof...(dims)>::make(vec_dims, v);
}

using namespace std;
//********************  dumps ************************//

template <typename T>
void dump(const T &data, vector<int> &iter)
{
  cout << data << " ";
}
template <typename T>
void dump(const vector<T> &data, vector<int> &iter)
{
  for (auto elem : data)
    dump(elem, iter);
  cout << endl;
}

template <typename T>
void dump(const vector<vector<T>> &data, vector<int> &iter)
{
  for (auto elem : iter)
  {
    cout << "[" << elem << "]";
  }
  cout << endl;
  for (int i = 0; i < data.size(); i++)
  {
    iter.push_back(i);
    dump(data[i], iter);
    iter.pop_back();
  }
  cout << endl;
}

template <typename T>
void dump(const vector<T> &data, int dummy)
{
  for (int i = 0; i < data.size(); i++)
  {
    cout << "[" << i << "] " << data[i] << endl;
  }
}
template <typename T>
void dump(const T &data)
{ // T : data, U = base type val.
  vector<int> iter;
  dump(data, iter);
}
///////////////////////////////////////////////

int main()
{
  int H, W, K;
  cin >> H >> W >> K;
  vector<string> field;
  vector<vector<int>> ruiseki(H + 1, vector<int>(W + 1, 0));
  for (int i = 0; i < H; i++)
  {
    string s;
    cin >> s;
    field.push_back(s);
    for (int j = 0; j < W; j++)
    {
      ruiseki[i + 1][j + 1] = ruiseki[i + 1][j] + ruiseki[i][j + 1] - ruiseki[i][j];
      if (field[i][j] == '1')
        ruiseki[i + 1][j + 1]++;
    }
  }
  auto count = [&](int x1, int x2, int y1, int y2) {
    return ruiseki[x2][y2] - ruiseki[x1][y2] - ruiseki[x2][y1] + ruiseki[x1][y1];
  };

  int ans = 1000000;
  for (int bit = 0; bit < (1 << (H - 1)); bit++)
  {
    vector<int> kireme = {0};
    for (int i = 0; i < H - 1; i++)
      if ((bit & (1 << i)))
        kireme.push_back(i + 1);
    kireme.push_back(H);
    auto calc = [&](int prev) {
      for (int j = prev + 1; j <= W; j++)
      {
        for (int i = 0; i < kireme.size() - 1; i++)
        {
          int before = kireme[i];
          int after = kireme[i + 1];
          if (K < count(before, after, prev, j))
          {
            return j - 1;
          }
        }
      }
      return W;
    };
    int cur = 0;
    int cnt = kireme.size() - 3;
    while (cur != W)
    {
      //cout << cur << endl;
      int prev = cur;
      cur = calc(cur);
      if (prev == cur)
      {
        cnt = 1000000;
        break;
      }
      cnt++;
    }
    //dump(kireme);
    chmin(ans, cnt);
  }
  cout << ans << endl;

  return 0;
}


Measured memory_bytes: 256