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

typedef long long ll;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<vector<vector<bool>>> vvvb;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<vpll> vvpll;
typedef vector<double> vd;
typedef vector<vd> vdd;

#define FOR(i,x,y) for(ll i=(ll)x; i<(ll)y; ++i)
#define REP(i,y) FOR(i, 0, y)
#define RFOR(i,x,y) for(ll i=(ll)x; i>=(ll)y; --i)
#define RREP(i,x) RFOR(i, x, 0)
#define ALL(a) a.begin(), a.end()
#define pb push_back
#define debug_print(x...) cerr << "line " << __LINE__ << " : "; debug_print_in(x);

template <typename First>
void debug_print_in(First first){
  cerr << first << endl;
  return;
}

template <typename First, typename... Rest>
void debug_print_in(First first, Rest... rest){
  cerr << first << " ";
  debug_print_in(rest...);
  return;
}

void IN(void){
  return;
}

template <typename First, typename... Rest>
void IN(First& first, Rest&... rest){
  cin >> first;
  IN(rest...);
  return;
}

template <typename First>
void OUT(First first){
  cout << first << endl;
  return;
}

template <typename First, typename... Rest>
void OUT(First first, Rest... rest){
  cout << first << " ";
  OUT(rest...);
  return;
}

template<class t, class u> void chmax(t&a,u b){if(a<b)a=b;};
template<class t, class u> void chmin(t&a,u b){if(a>b)a=b;};
int popcount(int t){return __builtin_popcount(t);} //GCC
int popcount(ll t){return __builtin_popcountll(t);} //GCC

template <typename T>
void vec_print(vector<T> VEC){
  REP(i, VEC.size()){
    cerr << VEC[i] << " ";
  }
  cerr << endl;
};

template <typename T>
void mat_print(vector<vector<T> > MAT){
  REP(i,MAT.size()){
    REP(j,MAT[i].size()){
      cerr << MAT[i][j] << " ";
    }
    cerr << endl;
  }
};

constexpr int INF = (1<<30);
constexpr ll INFLL = 1LL<<62;
constexpr long double EPS = 1e-12;
constexpr ll MOD = (ll)((1E+9)+7);

ll H, W, K;
vvll S;
vvll S_sum;

int main(){
  cin.tie(0); // cut the cin and cout (default, std::flush is performed after std::cin)
  ios::sync_with_stdio(false); // cut the iostream and stdio (DON'T endl; BUT "\n";)

  IN(H,W,K);
  S.resize(H);
  REP(i,H){
    string str;
    IN(str);
    S[i].resize(W, 0);
    REP(j,W){
      S[i][j] = (str[j]-'0');
    }
  }

  S_sum.resize(H+1, vll(W+1, 0));
  REP(i,H){
    REP(j,W){
      S_sum[i+1][j+1] = S[i][j] + S_sum[i+1][j] + S_sum[i][j+1] - S_sum[i][j];
    }
  }

  ll ans = INFLL;

  REP(index,1<<(H-1)){
    vpll tmp;
    ll pos=1;
    ll index_copy = index;
    ll ans_tmp=0;
    REP(j,H-1){
      if(index_copy%2==1){
        tmp.pb(make_pair(pos, j+1));
        pos = j+2;
        ans_tmp++;
      }
      index_copy>>=1;
    }
    tmp.pb(make_pair(pos, H));

    /*
    REP(i, tmp.size()){
      debug_print(tmp[i].first, tmp[i].second);
    }

    debug_print(index, ans_tmp);
    */

    vll K_sum(tmp.size(), 0);
    vll K_shift(tmp.size(), 0);

    FOR(pos, 1, W+1){
      REP(i, tmp.size()){
        K_shift[i] = 0;
        FOR(j, tmp[i].first, tmp[i].second+1){
          K_shift[i] += S[j-1][pos-1];
        }
      }

      bool flag = true;
      REP(i, tmp.size()){
        if(K_sum[i]+K_shift[i]>K){
          flag = false;
        }
        if(K_shift[i]>K){
          ans_tmp += H*W;
        }
      }

      if(flag){
        REP(i, tmp.size()) K_sum[i] += K_shift[i];
      }else{
        REP(i, tmp.size()) K_sum[i] = K_shift[i];
        ans_tmp++;
        //debug_print(pos);
        //vec_print(K_sum);
      }
    }

    chmin(ans, ans_tmp);
  }

  OUT(ans);

  return 0;
}


Measured memory_bytes: 384