/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author champon
 */

#include <iostream>
#include <fstream>

#include <bits/stdc++.h>
#define se cout << "test ok" << endl;
#define sp << " " <<
using namespace std;
using ll = long long;
using ld = long double;
ll mod = ll(1e9)+7;
ll inf = ll(1e18);
template<typename T1, typename T2> ll ceill(T1 a, T2 b){ return (a+b-1)/b; }
void ans(bool b, ostream& out=cout){ out << (b ? "Yes" : "No") << endl; }
void ans2(bool b, ostream& out=cout){ out << (b ? "YES" : "NO") << endl; }
template<typename T> inline bool chmax(T &a, T b){ if(a<b){a=b; return true;} return false; }
template<typename T> inline bool chmin(T &a, T b){ if(a>b){a=b; return true;} return false; }
template<typename A, size_t N, typename T> void Fill(A (&array)[N], const T &val){ fill((T*)array, (T*)(array+N), val); }
int dx[] = {1, 0, -1, 0, 1, -1, 1, -1};
int dy[] = {0, 1, 0, -1, 1, -1, -1, 1};

#ifndef INCLUDE_DEBUG_HPP
#define INCLUDE_DEBUG_HPP

#include <iostream>
#include <vector>
#include <map>
using namespace std;

namespace dbg {
    using ll = long long;

    class Debug {
    public:
        Debug(){}
        template<typename T>
        void vd(vector<T> vec);
        template<typename T>
        void vd2(vector<vector<T>> vec);
        template<typename T>
        void ad(T *arr, ll n);
        template<typename T, size_t N>
        void ad2(T (&arr)[N], ll n);
        template<typename T>
        void md(T mp);
    };

    template<typename T>
    void Debug::vd(vector<T> vec) {
        for (auto vv : vec) cout << vv << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::vd2(vector<vector<T>> vec) {
        for (auto vv : vec) for (auto vvv : vv) cout << vvv << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::ad(T *arr, ll n) {
        for (int i = 0; i <= n; i++) cout << arr[i] << " ";
        cout << endl;
    }

    template<typename T, size_t N>
    void Debug::ad2(T (&arr)[N], ll n) {
        for (int i = 0; i <= N; i++) for (int j = 0; j <= n; j++) cout << arr[i][j] << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::md(T mp) {
        cout << "key : value" << endl;
        for (auto const &m : mp) cout << m.first << " : " << m.second << endl;
        cout << endl;
    }

}

#endif //INCLUDE_DEBUG_HPP

using dbg::Debug;
Debug debug;


class EDividingChocolate {
public:
	void solve(std::istream& in, std::ostream& out) {
	    int h, w, k;
	    in >> h >> w >> k;
        vector<vector<char>> g(h, vector<char>(w));
        string s;
        for(int i=0; i<h; i++){
            in >> s;
            for(int j=0; j<w; j++){
                g[i][j] = s[j];
            }
        }

        ll res = inf;
        for(int m=0; m<(1<<h-1); m++){
            vector<int> cnt(h, 0);
            ll sum = 0;
            bool flg2 = true;
            for(int j=0; j<w; j++){
                ll now = 0;
                bool flg = true;
                for(int i=0; i<h; i++){
                    cnt[i] += (g[i][j] == '1');
                    now += cnt[i];
                    if(now > k) flg = false;
                    if(m & (1<<i)) now = 0;
                }
                if(!flg){
                    sum++;
                    now = 0;
                    for(int l=0; l<h; l++){
                        cnt[l] = (g[l][j] == '1');
                        now += cnt[l];
                        if(now > k) flg2 = false;
                        if(m & (1<<l)) now = 0;
                    }
                }
                if(!flg2) break;
            }
            if(!flg2) continue;
            chmin(res, sum + __builtin_popcount(m));
        }

        out << res << endl;
	}
};


int main() {
	EDividingChocolate solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 256