#include <bits/stdc++.h>
#include <boost/range/algorithm.hpp>
#include <boost/range/numeric.hpp>
#include <boost/integer/common_factor.hpp>
#include <boost/integer/common_factor_rt.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>

using std::string;
using std::vector;
using std::set;
using std::multiset;
using std::unordered_set;
using std::map;
using std::multimap;
using std::unordered_map;
using std::pair;
using std::cin;
using std::cout;

using boost::multiprecision::cpp_int;
using cpp_dec_float_1000 = boost::multiprecision::number<boost::multiprecision::cpp_dec_float<1000, boost::int128_type>>;

typedef uintmax_t    ull;
typedef intmax_t     ll;
typedef uint64_t     ul;
typedef uint32_t     ui;
typedef uint8_t      uc;

constexpr char CRLF  = '\n';
constexpr char SPACE = ' ';
constexpr char VECTOR_COUT_SEPARATOR = SPACE;
constexpr ll INF = 1000'000'007;
constexpr int MOD = 1000'000'007;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> std::ostream& operator<< (std::ostream& os, const std::vector<T>& vc) { for(auto it = vc.begin(); it != vc.end(); ++it) { if (std::next(it) == vc.end()) os << *it; else os << *it << VECTOR_COUT_SEPARATOR; } return os; }
template<class T1, class T2> inline std::ostream & operator<< (std::ostream & os, const std::pair<T1, T2> & p) { return os << p.first << ' ' << p.second; }
template<class T> T modinv(T a, T m) { T b = m, u = 1, v = 0; while (b) { T t = a / b; a -= t * b; boost::swap(a, b); u -= t * v; boost::swap(u, v); } u %= m; if (u < 0) u += m; return u; }
template<class T> inline bool isSosuu(const T& n) { if (n == T(1)) { return false; } for (T i = 2; i*i <= n; ++i) { if (n % i == 0) { return false; } } return true; }
template<class T> vector<vector<T>> split (const vector<T>& S, T delim) { vector<vector<T>> ret{}; vector<T> buff{}; for (auto c : S) { if (c == delim) { if (!buff.empty()) ret.push_back(buff); buff.clear(); } else { buff.push_back(c); } } if (!buff.empty()) ret.push_back(buff); return ret; }

struct Tmp
{
public:
  int H, W, K;
  vector<vector<int>> table;
  Tmp(int H, int W, int K)
  : H(H)
  , W(W)
  , K(K)
  , table(H, vector<int>(W, 0))
  {}

  ll operator()() {
    int itr = 1 << H;
    ll res = INF;
    while (itr) {
      auto r = task(itr);
      chmin(res, r);
      --itr;
    }
    return res;
  }

  ll task(int bit) {
    vector<vector<int>> table2;
    vector<int> buff(W);
    ll yokoNoSen{};
    for (int h = 0; h < H; ++h) {
      for (int w = 0; w < W; ++w) {
        buff[w] += table[h][w];
      }
      if (bit & (1 << h)) {
        if (h < H-1) {
          ++yokoNoSen;
        }
        table2.push_back(std::move(buff));
        buff = vector<int>(W, 0);
      }
    }
    table2.push_back(std::move(buff));

    int nH = table2.size();
    vector<int> sum(nH);
    ll imanokugiri{};
    ll tateNoSen{};
    for (int w = 0; w < W; ++w) {
      for (int h = 0; h < nH; ++h) {
        sum[h] += table2[h][w];
        ++imanokugiri;
      }
      auto m = boost::max_element(sum);
      if (*m > K) {
        int mm{};
        for (int h = 0; h < nH; ++h) {
          chmax(mm, table2[h][w]);
        }
        if (mm > K) {
          return INF;
        }
        if (imanokugiri == 1) {
          // 無理
          return INF;
        }
        imanokugiri = 1;
        ++tateNoSen;
        for (int h = 0; h < nH; ++h) {
          sum[h] = table2[h][w];
        }
      }
    }
    return yokoNoSen + tateNoSen;
  }
};

void solve(void)
{
  int H, W; cin >> H >> W;
  int K; cin >> K;
  Tmp t(H, W, K);
  for (int h = 0; h < H; ++h) {
    for (int w = 0; w < W; ++w) {
      char c; cin >> c;
      if (c == '1') {
        t.table[h][w] = 1;
      }
    }
  }
  auto ans = t();
  cout << ans << CRLF;
  return;
}

int main(void)
{
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(0);

  solve();

  return 0;
}


Measured memory_bytes: 384