#include "bits/stdc++.h"
using namespace std;
#define Rep(i,n) for(int i=0;i<(int)(n);i++)
#define For(i,n1,n2) for(int i=(int)(n1);i<(int)(n2);i++)
#define REP(i,n) for(ll i=0;i<(ll)(n);i++)
#define RREP(i,n) for(ll i=((ll)(n)-1);i>=0;i--)
#define FOR(i,n1,n2) for(ll i=(ll)(n1);i<(ll)(n2);i++)
#define RFOR(i,n1,n2) for(ll i=((ll)(n1)-1);i>=(ll)(n2);i--)
#define all(a)  (a).begin(),(a).end()
#define SORT(a) sort((a).begin(),(a).end())
#define oorret 0
#define oor(x) [&](){try{x;} catch(const out_of_range& oor){return oorret;} return x;}()
#define IOS cin.tie(0),ios::sync_with_stdio(false)
typedef long long ll;
typedef unsigned long long ull;
typedef std::pair<ll, ll> P;
template<typename T1, typename T2> inline bool chmin(T1& a, T2 b) { if (a > b) { a = b; return 1; }return 0; }
template<typename T1, typename T2> inline bool chmax(T1& a, T2 b) { if (a < b) { a = b; return 1; }return 0; }
template<class Type>struct is_vector : std::false_type {};
template<class ValueType, class Alloc>struct is_vector<std::vector<ValueType, Alloc>> : std::true_type {};
template <typename T> inline std::ostream& operator << (std::ostream& out, const std::vector<T>& v) {
	if (v.empty())return out;
	constexpr bool is_vector_v = is_vector<T>::value;
	if (is_vector_v)for (auto itr = v.begin(); itr != v.end();)out << (*itr), out << ((++itr != v.end()) ? "\n" : "");
	else for (auto itr = v.begin(); itr != v.end();)out << (*itr), out << ((++itr != v.end()) ? " " : "");
	return out;
}
inline void put() {}
template<class T> inline void put(const T& first) { std::cout << first << "\n"; }
template<class T, class... N> inline void put(const T& first, const N& ... rest) { std::cout << first << " "; put(rest...); }
inline void putn() {}
template<class T, class... N> inline void putn(const T& first, const N& ... rest) { std::cout << first << "\n"; putn(rest...); }

vector<int> demical(ll n, int k, int l = -1) {
	vector<int> rtn;
	if (l < 0) {
		while (n > 0) {
			rtn.push_back(n % k);
			n /= k;
		}
	} else {
		rtn.resize(l);
		REP(i, l) {
			rtn[i] = n % k;
			n /= k;
		}
	}
	return rtn;
}

int main() {
	IOS;
	int h, w, k;
	cin >> h >> w >> k;
	vector<string> s(h);
	vector<vector<int>> ch(h, vector<int>(w));
	REP(i, h) {
		cin >> s[i];
		REP(j, w) {
			if (s[i][j] == '0') {
				ch[i][j] = 0;
			} else {
				ch[i][j] = 1;
			}
		}
	}
	int t = 1 << (h - 1);
	int res = INT_MAX;
	REP(i, t) {
		auto d = demical(i, 2, h - 1);
		int tmp = 0;
		vector<int> p;
		bool flag2 = true;
		REP(j, d.size()) {
			if (d[j] == 1) {
				tmp++;
				p.push_back(j);
			}
		}
		int s = d.size();
		s++;
		p.push_back(h);
		vector<int> c1(s,0);
		vector<int> c2(s,0);
		REP(j, w) {
			int cnt = 0;
			REP(l, h) {
				if (ch[l][j] == 1) {
					c2[cnt]++;

				}
				if (p[cnt] == l) {
					cnt++;
				}
			}
			bool flag = true;
			REP(l, s) {
				if (c1[l] + c2[l] > k) {
					flag = false;
				}
				if (c2[l] > k) {
					flag2 = false;
				}
			}
			if (!flag) {
				tmp++;
				REP(l, s) {
					c1[l] = c2[l];
				}
			} else {
				REP(l, s) {
					c1[l] += c2[l];
				}
			}
			REP(l, s) {
				c2[l] = 0;
			}
			if (!flag2) {
				break;
			}
		}
		if (flag2) {
			chmin(res, tmp);
		}
	}
	put(res);
	return 0;
}

Measured memory_bytes: 256