#include <bits/stdc++.h>
#ifdef LOCAL_DEBUG
#define DEBUG 1
#define CERR if(DEBUG) cerr
#define MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,NAME,...) NAME
#define pr(...) CERR<<MACRO(__VA_ARGS__,pr10,pr9,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__)<<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define pr9(a,b,c,d,e,f,g,h,i) pr1(a)<<pr8(b,c,d,e,f,g,h,i)
#define pr10(a,b,c,d,e,f,g,h,i,j) pr1(a)<<pr9(b,c,d,e,f,g,h,i,j)
#define prArr(a){CERR<<(#a)<<"={";int i=0;for(auto t:(a))CERR<<(i++?", ":"")<<t;CERR<<"}"<<endl;}
#else
#define DEBUG 0
#define pr(...)
#define prArr(a)
#endif
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}
//INSERT ABOVE HERE

Int calc(const vector<string> &mp, Int l, Int r, Int K){
  Int h = mp.size();
  Int w = mp[0].size();
  Int num = 0;
  for(Int i=0;i<h;i++)
    for(Int j=l;j<=r;j++) {
      num += mp[i][j] == '1';
    }
  return num;
}

Int check(const vector<string> &mp, Int l, Int r, Int K, Int ofset){
  Int w = mp[0].size();
  if(r >= w) return 0;
  return ofset + calc(mp, l, r, K) <= K;
}


Int check(const vector<vector<string> >&mp, Int K){
  Int w = mp[0][0].size();
  Int res = 0;
  Int l = 0;
  Int n = mp.size();
  vector<Int> ofset(n);
  for(Int i=0;i<w;i++){
    Int flag = 1;
    for(Int j=0;j<n;j++)
      if(check(mp[j], i, i, K, ofset[j]) == 0) flag = 0;

    Int flag2 = 1;
    for(Int j=0;j<n;j++)
      if(check(mp[j], i, i+1, K, ofset[j]) == 0) flag2 = 0;

    for(Int j=0;j<n;j++) ofset[j] += calc(mp[j], i, i, K);

    if(flag == 0) return INF;
    //pr(l, i, res, flag, flag2);
    if(flag && !flag2) {
      res++;
      l = i+1;
      ofset = vector<Int>(mp.size(), 0);
    }
  }
  //pr(mp);
  //pr(res);
  return res - 1;
}

signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);

  Int H, W, K;
  cin>>H>>W>>K;

  vector<string> mp(H);
  cin>>mp;

  Int ans = INF;
  for(Int i=0;i<(1<<(H-1));i++){
    vector<string> A;
    vector<vector<string> > As;

    A.push_back(mp[0]);
    for(Int j=0;j<H-1;j++){
      if(i>>j&1){
        As.push_back(A);
        A.clear();
      }
      A.push_back(mp[j+1]);
    }
    if(!A.empty()) As.push_back(A);
    Int num_bit = __builtin_popcount(i);
    Int cost = check(As, K) + num_bit;
    Min(ans, cost);
  }
  cout<<ans<<endl;
  return 0;
}


Measured memory_bytes: 256