#include <iostream>
#include <iomanip>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <list>
#include <stack>
#include <queue>
#include <bitset>
#include <numeric>
#include <cassert>
#include <array>
#include <memory>
#include <random>
#include <functional>
#include <complex>
#ifdef DEBUG
#include "./Lib/debug.hpp"
#include "./Lib/Timer.hpp"
#include "./Lib/sample.hpp"
#else
#define dump(...)
#endif

/* (=＾o＾=) */
//#define int ll

/* macro */
#define FOR(i, b, e) for(ll i = (ll)(b); i < (ll)(e); ++i)
#define RFOR(i, b, e) for(ll i = (ll)(e-1); i >= (ll)(b); --i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) RFOR(i, 0, n)
#define REPC(x,c) for(const auto& x:(c))
#define REPI2(it,b,e) for(auto it = (b); it != (e); ++it)
#define REPI(it,c) REPI2(it, (c).begin(), (c).end())
#define RREPI(it,c) REPI2(it, (c).rbegin(), (c).rend())
#define REPI_ERACE2(it, b, e) for(auto it = (b); it != (e);)
#define REPI_ERACE(it, c) REPI_ERACE2(it, (c).begin(), (c).end())
#define ALL(x) (x).begin(),(x).end()
#define cauto const auto&
/* macro func */
template<class T>
inline void sort(T& t) { std::sort(ALL(t)); }
template<class T>
inline void rsort(T& t) { std::sort((t).rbegin(), (t).rend()); }
template<class T>
inline void unique(T& t) { (t).erase(unique((t).begin(), (t).end()), (t).end()); }
template<class T, class S>
inline bool chmax(T& t, const S& s) { if (s > t) { t = s; return true; } return false; }
template<class T, class S>
inline bool chmaxE(T& t, const S& s) { if (s >= t) { t = s; return true; } return false; }
template<class T, class S>
inline bool chmin(T& t, const S& s) { if (s < t) { t = s; return true; } return false; }
inline void BR() { std::cout << "\n"; }

/* type define */
using ll = long long;
using PAIR = std::pair<ll, ll>;
using VS = std::vector<std::string>;
using VL = std::vector<long long>;
using VVL = std::vector<VL>;
using VVVL = std::vector<VVL>;
using VD = std::vector<double>;
template<class T>
using V = std::vector<T>;

/* using std */
using std::cout;
constexpr char endl = '\n';
using std::cin;
using std::pair;
using std::string;
using std::stack;
using std::queue;
using std::vector;
using std::list;
using std::map;
using std::unordered_map;
using std::multimap;
using std::unordered_multimap;
using std::set;
using std::unordered_set;
using std::unordered_multiset;
using std::multiset;
using std::bitset;
using std::priority_queue;
using std::tuple;

/* constant value */
constexpr ll MOD = 1000000007;
//constexpr ll MOD = 998244353;
//constexpr ll MOD = 2;

/* Initial processing  */
struct Preprocessing { Preprocessing() { std::cin.tie(0); std::ios::sync_with_stdio(0); }; }_Preprocessing;

/* Remove the source of the bug */
inline signed pow(signed, signed) { assert(false); return -1; }

/* define hash */
namespace std {
template <>	class hash<std::pair<ll, ll>> { public:	size_t operator()(const std::pair<ll, ll>& x) const { return hash<ll>()(1000000000 * x.first + x.second); } };
}

/* input */
template<class T> std::istream& operator >> (std::istream& is, vector<T>& vec) { for (T& x : vec) is >> x; return is; }

//=============================================================================================

class CumulativeSum2D {
private:
	const ll h;
	const ll w;
	std::vector<std::vector<ll>> sumList;
public:

	CumulativeSum2D() = delete;
	CumulativeSum2D(const std::vector<std::vector<ll>>& v) :
		h(v.size()),
		w(v[0].size()),
		sumList(h + 1, std::vector<ll>(w + 1)) {
		for (ll i = 0; i < h; ++i) for (ll j = 0; j < w; ++j) {
			sumList[i + 1][j + 1] = v[i][j];
		}
		for (ll i = 0; i < h; ++i) for (ll j = 0; j < w + 1; ++j) {
			sumList[i + 1][j] += sumList[i][j];
		}
		for (ll i = 0; i < h + 1; ++i) for (ll j = 0; j < w; ++j) {
			sumList[i][j + 1] += sumList[i][j];
		}
	}
	ll get(ll y, ll x) {
		return sumList[y + 1][x + 1];
	}
	ll get(ll y1, ll x1, ll y2, ll x2) {
		if (y2 < y1 || x2 < x1) { return 0LL; }
		x1 = std::max(x1, 0LL); y1 = std::max(y1, 0LL);
		y2 = std::min(y2, h - 1); x2 = std::min(x2, w - 1);
		return sumList[y2 + 1][x2 + 1] + sumList[y1][x1] -
			sumList[y2 + 1][x1] - sumList[y1][x2 + 1];
	}
};


auto bitC(ll i) {
	ll cnt = 0;
	while (i > 0) {
		if (i & 1) { ++cnt; }
		i >>= 1;
	}
	return cnt;
}

signed main() {
	ll h, w, k;
	cin >> h >> w >> k;
	VS mp(h);
	cin >> mp;

	VVL mpv(h, VL(w));
	REP(i, h)REP(j, w) {
		mpv[i][j] = (mp[i][j] - '0');
	}
	ll ans = 1e10;
	auto sum = CumulativeSum2D(mpv);

	REP(i, 1LL << (h - 1)) {
		VL t; t.reserve(h + 1);
		t.emplace_back(-1);
		REP(j, h - 1)if ((i >> j) & 1) { t.emplace_back(j); }
		t.emplace_back(h - 1);
		ll tn = t.size();

		bool ok = true;

		ll cnt = bitC(i);
		ll fromW = 0;
		REP(pw, w) {
			if (!ok) { break; }
			REP(jh, tn - 1) {
				//cout << t[jh] + 1 << " " << t[jh + 1] << " , " << fromW << endl;
				if (sum.get(t[jh] + 1, fromW, t[jh + 1], pw) > k) {
					fromW = pw;
					++cnt;
					if (sum.get(t[jh] + 1, pw, t[jh + 1], pw) > k) {
						//dump("out");
						ok = false;
					}
					break;
				}
			}
		}

		if (ok) {
			chmin(ans, cnt);
		}
	}

	cout << ans << endl;
}


Measured memory_bytes: 456