#include <bits/stdc++.h>
using namespace std;

// #include <boost/algorithm/string/join.hpp>
// using boost::algorithm::join;
//
// #include "boost/multi_array.hpp"
// boost::multi_array<int, 3> dp;
// dp.resize(boost::extents[10][9][8]);
//
// #include <boost/multiprecision/cpp_dec_float.hpp>  // quite slowly
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision; // mp::log10
// using bigint = boost::multiprecision::cpp_int;
// using rational = mp::cpp_rational;  // double d = static_cast<double>(r);

typedef long long ll;
typedef unsigned long long ull;
//typedef long double ld;

typedef map<int, int> mii;
typedef pair<int, int> pii;
typedef pair<double,double> pdd;
typedef set<int> si;
typedef set<ll> sll;
typedef vector<int> vi;
typedef vector<pair<int, int > > vii;
typedef vector<ll> vll;
typedef vector<double> vd;
typedef vector<vi> vvi;
typedef vector<vii> vvii;
typedef vector<vll> vvll;

//constexpr ll MOD = 1e9+7;
template<typename T>constexpr auto inf=numeric_limits<T>::max()/4;
constexpr int INF=inf<int>;
constexpr ll LINF=inf<ll>;

#define Sort(x) sort(begin(x),end(x))
#define Reverse(x) reverse(begin(x),end(x))
#define ABS(a,b) ((a)<(b)?(b)-(a):(a)-(b))
#define nbit(n) (1ull<<(n))
#define _ol3(_1,_2,_3,name,...)name
#define _rep(i,n)_repi(i,0,n)
#define _repi(i,a,b)for(int i=a,i##_l=(b);i<i##_l;++i)
#define REP(...) _ol3(__VA_ARGS__,_repi,_rep,)(__VA_ARGS__)
#define REPR(i,n) for(int i=n-1;i>=0;--i)
#define REPA(i,v) REP(i,(v).size())

// sort(begin(x), end(x), greater<ll>());
// lower_bound(a.begin(), a.end(), val): a[i] ≥ valである最左の位置
// upper_bound(a.begin(), a.end(), val): a[i] > valである最左の位置

// Returns minimum of a and b.
// If a is less b, a is set to b.
template<typename T>
T chmin(T& a, T b) {
  if (a > b) {
    a = b;
  }
  return a;
}

// Returns maximum of a and b.
// If a is less b, a is set to b.
template<typename T>
T chmax(T& a, T b) {
  if (a < b) {
    a = b;
  }
  return a;
}

template <typename T>
void dumpContents(const T& v, const string& msg)
{
  cerr << "### " << msg << " ###\n";
  for (const auto& x : v) {
    cerr << x << " ";
  }
  cerr << endl;
}

struct before_main_function {
  before_main_function() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout<<setprecision(20)<<fixed;
#define endl "\n"
  }
} before_main_function;

// ========== end of template ==========
constexpr int H_MAX = 10;
constexpr int W_MAX = 1000;

int H, W, K;

int G[H_MAX][W_MAX] = {{0}};

int solve(uint hmask) {
  int ret = 0;

  // hのline
  REP(i, H-1) {
    if (hmask >> i & 1u) ret++;
  }

//cerr << "hmask " << hmask << endl;

  // wのline
  vi s(H_MAX);
  REP(w, W) {

//cerr << "w " << w << " ret " << ret << endl;

    vi tmp = s;

    int i = 0;
    REP(h, H) {
      tmp[i] += G[h][w];
      if ((hmask >> h) & 1u) i++;
    }
//dumpContents(tmp, "tmp");

    // check tmp
    bool ok = true;
    for (auto x : tmp) {
      if (x > K) {
        ok = false;
        break;
      }
    }

    if (ok) {
      //cerr << "   ok" << endl;
      REP(i, H_MAX) {
        s[i] = tmp[i];
      }
      continue;
    }

    ret++;
    fill(begin(s), end(s), 0);

    i = 0;
    REP(h, H) {
      s[i] += G[h][w];
      if ((hmask >> h) & 1u) i++;
    }

    // check s
    ok = true;
    for (auto x : s) {
      if (x > K) {
        ok = false;
        break;
      }
    }

    if (!ok) return INF;
  }

  return ret;
}

int main(int argc, char** argv) {
  cin >> H >> W >> K;

  REP(i, H) {
    REP(j, W) {
      char c;
      cin >> c;
      G[i][j] = c-'0';
    }
  }

  /*
  REP(i, H) {
    REP(j, W) {
      cerr << G[i][j];
    }
    cerr << endl;
  }
  */


  int ret = INF;
//cerr << "ret " << ret << endl;
  for (uint hmask=0; hmask < nbit(H-1); hmask++) {
    chmin(ret, solve(hmask));
  }

  cout << ret << endl;

  return 0;
}

/* vim: set expandtab shiftwidth=2 softtabstop=2 : */


Measured memory_bytes: 256