// template version 1.15
using namespace std;
#include <bits/stdc++.h>

// varibable settings
#define int long long
const int INF=1e18;

// define basic macro {{{
#define _overload3(_1,_2,_3,name,...) name
#define _rep(i,n) repi(i,0,n)
#define repi(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define _rrep(i,n) rrepi(i,0,n)
#define rrepi(i,a,b) for(int i=(int)((b)-1);i>=(int)(a);--i)
#define rrep(...) _overload3(__VA_ARGS__,rrepi,_rrep,)(__VA_ARGS__)
#define each(i,a) for (auto&& i : a)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(a, b, c) make_tuple(a, b, c)
#define ub upper_bound
#define lb lower_bound
#define posl(A, x) (lower_bound(all(A), x)-A.begin())
#define posu(A, x) (upper_bound(all(A),x)-A.begin())
template<class T> inline void chmax(T &a, const T &b) { if((a) < (b)) (a) = (b); }
template<class T> inline void chmin(T &a, const T &b) { if((a) > (b)) (a) = (b); }

#define divceil(a,b) ((a)+(b)-1)/(b)
#define is_in(x, a, b) ((a)<=(x) && (x)<(b))
#define uni(x) sort(all(x));x.erase(unique(all(x)),x.end())
#define slice(l, r) substr(l, r-l)

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long double ld;
typedef pair<int,int> pii;
typedef tuple<int,int,int> iii;

template<typename T> using PQ = priority_queue<T, vector<T>, greater<T>>;
struct Fast { Fast(){ std::cin.tie(0); ios::sync_with_stdio(false); } } fast;

#if defined(PCM) || defined(LOCAL)
    #include "lib/dump.hpp"
#else
    #define dump(...) 42
    #define dump_1d(...) 42
    #define dump_2d(...) 42
    #define cerrendl 42
#endif
//}}}

signed main() {
    int h,w,k;cin>>h>>w>>k;

    vvi e(h, vi(w));
    rep(i, h){
        string s;cin>>s;
        rep(j, w){
            e[i][j] = (s[j]=='1' ? 1 : 0);
        }
    }
    dump_2d(e, h, w);

    int ans = INF;
    rep(mask, 1<<(h-1)){
        cerrendl;
        int res = 0;

        vi vmask(h-1);
        int c = 0;
        rep(i, h-1){
            if (mask&(1<<i)){
                vmask[i] = 1;
                c++;
            }
        }
        res += c;

        dump(vmask);
        vector<int> pos(h);
        int cur = 0;
        rep(i, h-1){
            if (vmask[i]){ cur++; }
            pos[i+1] = cur;
        }
        dump(pos);

        vector<int> cnt(1+c);
        bool valid = true;
        rep(j, w){
            vector<int> tmp(1+c);
            rep(i, h){
                if (e[i][j]){
                    tmp[pos[i]]++;
                    if (tmp[pos[i]]>k){
                        valid = false;
                        break;
                    }
                }
            }
            if (!valid) break;
            dump(tmp);
            vector<int> newcnt(cnt);
            bool over = false;
            rep(i, 1+c){
                newcnt[i] += tmp[i];
                if (newcnt[i]>k){
                    over = true;
                    break;
                }
            }
            if (over){
                res++;
                cnt = tmp;
            }
            else{
                cnt = newcnt;
            }
        }
        if (valid) chmin(ans, res);
    }
    cout << ans << endl;

    return 0;
}


Measured memory_bytes: 384