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

// macros
#define pb emplace_back
#define mk make_pair
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define irep(itr, st) for(auto itr = (st).begin(); itr != (st).end(); ++itr)
#define irrep(itr, st) for(auto itr = (st).rbegin(); itr != (st).rend(); ++itr)
#define whole(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define bit(n) (1LL<<(n))
// functions
template <typename T> void unique(T& c){c.erase(std::unique(c.begin(), c.end()), c.end());}
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}
template <typename T> ostream &operator<<(ostream &os, const vector<T>& vec){for(int i = 0; i < vec.size(); i++){ os << vec[i]; if(i + 1 != vec.size())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T>& st){for(auto itr = st.begin(); itr != st.end(); ++itr){ os << *itr; auto titr = itr; if(++titr != st.end())os << " ";} return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p){os << "(" << p.first << ", " << p.second << ")"; return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << "(" << itr->first << ", " << itr->second << ")"; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const unordered_map<T1, T2> &mp){for(auto itr = mp.begin(); itr != mp.end(); ++itr){ os << "(" << itr->first << ", " << itr->second << ")"; auto titr = itr; if(++titr != mp.end())os << " "; } return os;}
//  types
using ll = long long int;
using P = pair<int, int>;
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1000000007;
const int dx[4] = {-1, 0, 1, 0};
const int dy[4] = {0, -1, 0, 1};
// io
struct fast_io{
  fast_io(){ios_base::sync_with_stdio(false); cin.tie(nullptr); cout << fixed << setprecision(20);}
} fast_io_;

string ss[11];
int h, w, k;
int s[11][1010];
vector<int> v;
int res = inf;

void dfs(int i){
  if(i == h){
    vector<int> cnt(sz(v), 0);
    int tmp = 0;
    auto add = [&](int j){
      int c = 0;
      rep(l, sz(v)){
        for(int m = c; m < c + v[l]; m++){
          cnt[l] += s[m][j];
        }
        c = c + v[l];
      }
    };
    auto remove = [&](int j){
      int c = 0;
      rep(l, sz(v)){
        for(int m = c; m < c + v[l]; m++){
          cnt[l] -= s[m][j];
        }
        c = c + v[l];
      }
    };
    auto ok = [&](){
      for(auto e: cnt){
        if(e > k)return false;
      }
      return true;
    };
    for(int j = 0; j < w; j++){
      add(j);
      if(!ok()){
        rep(l, sz(v))cnt[l] = 0;
        ++tmp;
        add(j);
        if(!ok())return;
      }
    }
    /*
    cerr << v << " " << tmp << endl;
    */
    tmp = tmp + sz(v) - 1;
    chmin(res, tmp);
    return;
  }
  for(int j = 1; i + j <= h; j++){
    v.pb(j);
    dfs(i + j);
    v.pop_back();
  }
}

int main(int argc, char const* argv[])
{
  cin >> h >> w >> k;
  rep(i, h)cin >> ss[i];
  rep(i, h)rep(j, w){
    s[i][j] = int(ss[i][j] - '0');
  }
  dfs(0);
  cout << res << endl;
  return 0;
}


Measured memory_bytes: 384