#include <bits/stdc++.h>
using namespace std;
#define INF 1<<30
#define endl '\n'
#define maxn 1000005
#define FASTIO ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
typedef long long ll;
const double PI = acos(-1.0);
const ll mod = 1e9 + 7;

inline void normal(ll &a) { a %= mod; (a < 0) && (a += mod); }
inline ll modMul(ll a, ll b) { a %= mod, b %= mod; normal(a), normal(b); return (a * b) % mod; }
inline ll modAdd(ll a, ll b) { a %= mod, b %= mod; normal(a), normal(b); return (a + b) % mod; }
inline ll modSub(ll a, ll b) { a %= mod, b %= mod; normal(a), normal(b); a -= b; normal(a); return a; }
inline ll modPow(ll b, ll p) { ll r = 1; while (p) { if (p & 1) r = modMul(r, b); b = modMul(b, b); p >>= 1; } return r; }
inline ll modInverse(ll a) { return modPow(a, mod - 2); }
inline ll modDiv(ll a, ll b) { return modMul(a, modInverse(b)); }


/* for Random Number generate
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
*/
///**
template < typename F, typename S >
ostream& operator << ( ostream& os, const pair< F, S > & p ) {
  return os << "(" << p.first << ", " << p.second << ")";
}

template < typename T >
ostream &operator << ( ostream & os, const vector< T > &v ) {
  os << "{";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if ( it != v.begin() ) os << ", ";
    os << *it;
  }
  return os << "}";
}

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

template < typename F, typename S >
ostream &operator << ( ostream & os, const map< F, S > &v ) {
  os << "[";
  for (auto it = v.begin(); it != v.end(); ++it) {
    if ( it != v.begin() ) os << ", ";
    os << it -> first << " = " << it -> second ;
  }
  return os << "]";
}

#define dbg(args...) do {cerr << #args << " : "; faltu(args); } while(0)

clock_t tStart = clock();
#define timeStamp dbg("Execution Time: ", (double)(clock() - tStart)/CLOCKS_PER_SEC)

void faltu () { cerr << endl; }

template <typename T>
void faltu( T a[], int n ) {
  for (int i = 0; i < n; ++i) cerr << a[i] << ' ';
  cerr << endl;
}

template <typename T, typename ... hello>
void faltu( T arg, const hello &... rest) { cerr << arg << ' '; faltu(rest...); }
// Program showing a policy-based data structure.
#include <ext/pb_ds/assoc_container.hpp> // Common file 
#include <ext/pb_ds/tree_policy.hpp>
#include <functional> // for less 
#include <iostream>
using namespace __gnu_pbds;
using namespace std;
// GNU link : https://goo.gl/WVDL6g
typedef tree<int, null_type, less_equal<int>, rb_tree_tag,
        tree_order_statistics_node_update>
        new_data_set;

// find_by_order(k) – ফাংশনটি kth ordered element এর একটা পয়েন্টার রিটার্ন করে। অর্থাৎ তুমি চাইলেই kth ইন্ডেক্সে কি আছে, সেটা জেনে ফেলতে পারছো!
// order_of_key(x) – ফাংশনটি x এলিমেন্টটা কোন পজিশনে আছে সেটা বলে দেয়।

//*//**___________________________________________________**/

const int H = 15;
const int W  = 1010;
string g[H];
vector<int> dp[W];
int h, w, k;
int ans;

int main()
{
  FASTIO
  /*
#ifndef ONLINE_JUDGE
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
  freopen("error.txt", "w", stderr);
#endif
//*/
  cin >> h >> w >> k;
  for (int i = 1; i <= h; i++) {
    cin >> g[i];
    g[i] = "#" + g[i];
  }

  ans = mod;
  for (int mask = 0; mask < (1 << (h - 1)); mask++) {
    int cnt = __builtin_popcount(mask);
    vector<int> cur(cnt + 1, 0);
    int x = mask | (1 << (h - 1));
    int flag = 1;
    for (int j = 1; j <= w; j++) {
      int ok = 1;
      vector<int> extra;
      int y = 0;
      for (int i = 1; i <= h; i++) {
        y += (g[i][j] == '1');
        if ((x >> (i - 1)) & 1) {
          extra.emplace_back(y);
          y = 0;
        }
      }
      for (auto it : extra) {
        if (it > k) {
          flag = 0;
          break;
        }
      }
      if (flag == 0) break;
      for (int i = 0; i < (int)extra.size(); i++) {
        cur[i] += extra[i];
        if (cur[i] > k) {
          ok = 0;
          break;
        }
      }

      if (!ok) {
        cnt++;
        cur = extra;
      }
    }
    if (flag) ans = min(ans, cnt);
  }
  cout << ans << endl;
  return 0;
}

Measured memory_bytes: 384