#include <iostream>
#include <algorithm>

#include <bitset>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <stack>
#include <queue>
#include <deque>
#include <cstring>
#include <string>
#include <utility>
#include <array>
#include <complex>
#include <valarray>

#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>

#include<memory>


using namespace std;
#define int long long

typedef long long ll;
typedef unsigned long long ull;
//typedef unsigned __int128 HASH;
typedef pair<int,int> pii;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pullull;
typedef pair<ll,int> plli;
typedef pair<double,int> pdi;
typedef pair<long double, int> pdbi;
typedef pair<int,pii> pipii;
typedef pair<ll,pll> plpll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<pii> vpii;
typedef vector<vector<int>> mat;

#define rep(i,n) for (int i=0;i<(n);i++)
#define rep2(i,a,b) for (int i=(a);i<(b);i++)
#define rrep(i,n) for (int i=(n);i>0;i--)
#define rrep2(i,a,b) for (int i=(a);i>b;i--)
#define fi first
#define se second
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()

template <class T>
using MINPQ = std::priority_queue<T, vector<T>, greater<T>>; //MINPQ<pii> pq;
template <class T>
using MAXPQ = std::priority_queue<T>;


const ll hmod1 = 999999937;
const ll hmod2 = 1000000000 + 9;
const int INF = 1<<30;
const ll INFLL = 1LL<<62;
const double EPS = 1e-12;
//const ll mod = 1000000000 + 7;
const ll mod = 998244353;
const int dx4[4] = {1, 0, -1, 0};
const int dy4[4] = {0, 1, 0, -1};
const int dx8[8] = {1, 1, 1, 0, 0, -1, -1, -1};
const int dy8[8] = {0, 1, -1, 1, -1, 0, 1, -1};
const long double pi = 3.141592653589793;

#define addm(X, Y) (X) = ((X) + ((Y) % mod) + mod) % mod
#define inside(y, x, h, w) (0 <= (y) && (y) < (h) && 0 <= (x) && (x) < (w)) ? true : false
#define stand(B, i) (((B>>i) & 1) == 1) ? true : false //Bのi番目のbitが1ならtrue, 0ならfalse

//debug
#define DEBUG

#define DUMPOUT cerr

#ifdef DEBUG
#define dump(...) DUMPOUT<<#__VA_ARGS__<<" :["<<__FUNCTION__<<":"<<__LINE__<<"]"<<endl; DUMPOUT<<"    "; dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

void dump_func() {DUMPOUT << endl;};

template <class Head, class... Tail> void dump_func(Head&& head, Tail&&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) == 0) DUMPOUT << " ";
    else DUMPOUT << ", ";
    dump_func(std::move(tail)...);
}

//ostream
template<typename T> ostream& operator << (ostream& os, vector<T>& vec) {
    os << "["; for (int i = 0; i<vec.size(); i++) os << vec[i] << (i + 1 == vec.size() ? "" : ", "); os << "]";
    return os;
}

template<typename T, typename U> ostream& operator << (ostream& os, pair<T, U>& pair_var) {
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}

template<typename T, typename U> ostream& operator << (ostream& os, map<T, U>& map_var) {
    os << "[";
    for (auto itr = map_var.begin(); itr != map_var.end(); itr++) {
        os << "(" << itr->first << ", " << itr->second << ")"; itr++;  if(itr != map_var.end()) os << ", "; itr--;
    }
    os << "]";
    return os;
}

template<typename T> ostream& operator << (ostream& os, set<T>& set_var) {
    os << "[";
    for (auto itr = set_var.begin(); itr != set_var.end(); itr++) {
        os << *itr; ++itr; if(itr != set_var.end()) os << ", "; itr--;
    }
    os << "]";
    return os;
}

int h, w;
int K;
string g[10];
int sum[10][1000 + 5];

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> h >> w >> K;
  rep(i, h) {
    cin >> g[i];
    rep(j, w) {
      sum[i][j] = g[i][j] - '0';
    }
  }
  rep(i, h)rep(j, w) {
    if (i == 0 && j == 0) continue;
    if (i == 0) {
      sum[i][j] += sum[i][j-1];
    }
    if (j == 0) {
      sum[i][j] += sum[i-1][j];
    }
    if (i > 0 && j > 0) {
      sum[i][j] += sum[i-1][j] + sum[i][j-1] - sum[i-1][j-1];
    }
    //cerr << i << " " << j << " " << sum[i][j] << endl;
  }
  int ans = INF;
  rep(i, 1<<(h-1)) {
    int pre = -1;
    vector<vector<int>> block;
    int cnt = 0;
    rep(j, h-1) {
      if (i >> j & 1) {
        cnt++;
        vector<int> tmp;
        rep2(k, pre+1, j+1) {
          tmp.push_back(k);
        }
        block.push_back(tmp);
        pre = j;
      }
    }
    vector<int> tmp;
    rep2(k, pre+1, h) {
      tmp.push_back(k);
    }
    block.push_back(tmp);
    int n = block.size();
    bool ok = true;
    int prek = -1;
    rep(k, w) {
      rep(j, n) {
        int cost = 0;
        int d = block[j].back();
        if (prek == -1 && j == 0) {
          cost = sum[d][k];
        }
        else if (prek == -1) {
          int pred = block[j-1].back();
          cost = sum[d][k] - sum[pred][k];
        }
        else if (j == 0) {
          cost = sum[d][k] - sum[d][prek];
        }
        else {
          int pred = block[j-1].back();
          cost = sum[d][k] + sum[pred][prek] - sum[pred][k] - sum[d][prek];
        }
        if (cost > K) {
          if (k - prek == 1) {
            ok = false;
            break;
          }
          cnt++;
          prek = k-1;
        }
      }
      if (!ok) {
        cnt = INF;
        break;
      }
    }
    //dump(block);
    //cerr << cnt << endl;
    ans = min(ans, cnt);
  }
  cout << ans << endl;
}


Measured memory_bytes: 384