#include <algorithm>
#include <bitset>
#include <cmath>
#include <complex>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <stack>
#include <string>
#include <vector>

const int MOD = 1e9 + 7;
const int iINF = 2147483647 / 2;
const long long int llINF = 9223372036854775807 / 2;

using namespace std;
using ll = long long int;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vvvl = vector<vector<vector<ll>>>;

typedef pair<ll, ll> pll;
bool paircomp(const pll &a, const pll &b) {
  if (a.first == b.first)
    return a.second < b.second;
  return a.first < b.first;
}

#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define AUTO(i, m) for (auto &i : m)
#define ALL(a) (a).begin(), (a).end()
#define MAX(vec) *std::max_element(vec.begin(), vec.end())
#define MIN(vec) *std::min_element(vec.begin(), vec.end())
#define ARGMAX(vec)                                                            \
  std::distance(vec.begin(), std::max_element(vec.begin(), vec.end()))
#define ARGMIN(vec)                                                            \
  std::distance(vec.begin(), std::min_element(vec.begin(), vec.end()))
#define REV(T) greater<T>()
#define PQ(T) priority_queue<T, vector<T>, greater<T>>
#define VVL(a, b, c) vector<vector<ll>>(a, vector<ll>(b, c))
#define VV(T, a, b, c) vector<vector<T>>(a, vector<T>(b, c))
#define VVVL(a, b, c, d)                                                       \
  vector<vector<vector<ll>>>(a, vector<vector<ll>>(b, vector<ll>(c, d)))
#define VVV(T, a, b, c, d)                                                     \
  vector<vector<vector<T>>>(a, vector<vector<T>>(b, vector<T>(c, d)))
#define SP(a) setprecision(a)
#define SQRT(a) sqrt((long double)(a))
#define DPOW(a, b) pow((long double)(a), (long double)(b))
#define UNIQUE(vec)                                                            \
  do {                                                                         \
    sort(ALL((vec)));                                                          \
    (vec).erase(std::unique(ALL((vec))), (vec).end());                         \
  } while (0)

ll POW(ll n, ll m) {
  if (m == 0) {
    return 1;
  } else if (m % 2 == 0) {
    ll tmp = POW(n, m / 2);
    return (tmp * tmp);
  } else {
    return (n * POW(n, m - 1));
  }
}

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ll H, W, K;
  cin >> H >> W >> K;
  vector<string> S(H);
  REP(i, H) cin >> S[i];
  ll ans = llINF;
  vvl SS = VVL(H + 1, W + 1, 0);

  REP(i, H)
  REP(j, W)
  SS[i + 1][j + 1] = (S[i][j] - '0') + SS[i + 1][j] + SS[i][j + 1] - SS[i][j];
  /*REP(i, H + 1) {
    REP(j, W + 1) cout << SS[i][j] << ",";
    cout << endl;
  }*/
  REP(i, 1 << (H - 1)) {
    ll now = 0;
    ll score = 0;
    ll po = i;
    vector<pll> yo;
    REP(k, H - 1) {
      if (po % 2 == 1) {
        yo.push_back({now, k});
        now = k + 1;
        score++;
      }
      po /= 2;
    }
    yo.push_back({now, H - 1});
    // cout << yo.size() << endl;
    for (ll j = 0; j < W;) {
      ll s = j;
      bool ok = true;

      AUTO(y, yo) {
        if (SS[y.second + 1][s + 1] - SS[y.second + 1][s] - SS[y.first][s + 1] +
                SS[y.first][s] >
            K) {
          ok = false;
        }
      }
      if (!ok) {
        score = llINF;
        break;
      }
      ok = true;
      while (ok && j < W - 1) {
        AUTO(y, yo) {
          if (SS[y.second + 1][j + 2] - SS[y.second + 1][s] -
                  SS[y.first][j + 2] + SS[y.first][s] >
              K) {
            ok = false;
          }
        }
        if (ok)
          j++;
      }
      score++;
      j++;
    }
    score--;
    ans = min(ans, score);
  }
  cout << ans << endl;
  return 0;
}


Measured memory_bytes: 384