#include <bits/stdc++.h>
#include <random>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef double ld;
//#define int ll
typedef vector<char> vc;
typedef vector<vc> vvc;
typedef vector<vvc> vvvc;
typedef pair<int, int> pii;
typedef pair<pii, pii> piii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii> vpi;
typedef vector< vi > vvi;
typedef vector< vvi > vvvi;
typedef vector<short> vs;
typedef vector<vs> vvs;
typedef vector<vvs> vvvs;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<ld> vld;
typedef vector<vld> vvld;
typedef vector<vvld> vvvld;
typedef vector<string> vst;
typedef vector<vst> vvst;
typedef pair<ld, ld> pld;

#define inmin(a, b) a = min(a, (b))
#define inmax(a, b) a = max(a, (b))
#define ALL(a) a.begin(),a.end()
#define RALL(a) a.rbegin(),a.rend()
#define sqr(x) ((x) * (x))
#define fori(i, n) for(int i = 0; i < int(n); ++i)
#define SZ(a) ((int)((a).size()))
#define triple(T) tuple<T, T, T>
#define quad(T) tuple<T, T, T, T>
#define watch(x) cerr << (#x) << " = " << (x) << endl;

#ifdef RUS_HOME
#define cerr cout
#else
#define cerr if (false) cerr
#endif

const double PI = 2 * acos(0.0);
#define rand shittttty_shit
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());

const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";


template <class T0, class T1>
inline ostream & operator << (ostream &out, pair<T0, T1> &a) {
    return out << "{" << a.first << ", " << a.second << "}";
}

template <class T0, class T1>
inline istream & operator >> (istream &in, pair<T0, T1> &a) {
    return in >> a.first >> a.second;
}

template <class T0, class T1, class T2>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << "}";
}

template <class T0, class T1, class T2, class T3>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2, T3> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << ", " <<  get<3>(a) << "}";
}

template<class T>
inline ostream & operator << (ostream &out, vector<T> &a) {
    out << "[";
    fori (i, a.size())
        out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
    return out;
}


void smain();



signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
#ifdef RUS_HOME
    freopen("input", "r", stdin);
    clock_t start = clock();
#endif
    cout << setprecision(12) << fixed;
    smain();
#ifdef RUS_HOME
    cout << "\n\n\n\nTOTAL EXECUTION TIME: " << float( clock () - start ) /  CLOCKS_PER_SEC << endl;
#endif
    return 0;
}

const int MOD = 998244353,N=3001;
int pref[10][N];

int get(int l1,int r1){
    if(l1<0||r1<0){
        return 0;
    }
    return pref[l1][r1];
}

int get(int l1,int r1,int l2,int r2){
    return get(l2,r2)-get(l1-1,r2)-get(l2,r1-1)+get(l1-1,r1-1);
}

void smain(){

    int h,w,k;
    cin>>h>>w>>k;
    vector<string>s(h);
    for(int i=0;i<h;i++){
        cin>>s[i];
        int pr=0;
        for(int j=0;j<w;j++){
            pr+=(s[i][j]=='1');
            pref[i][j]=pr;
            if(i)
                pref[i][j]+=pref[i-1][j];
        }
    }
    int ans=1e9;
    int cnt=(1<<(h-1));
    for(int i=0;i<cnt;i++){
        int mask=i;
        vi kek={-1};
        int cur=0;
        for(int j=0;j<h-1;j++){
            if(mask&(1<<j)){
                kek.push_back(j);
                cur++;
            }
        }
        kek.push_back(h-1);


        int lst=0;
        bool flag=true;
        for(int m=0;m<w;m++){
            for(int j=1;j<SZ(kek);j++) {
                if (get(kek[j - 1] + 1, m, kek[j], m) > k){
                    flag=false;
                }
                if (get(kek[j - 1] + 1, lst, kek[j], m) > k) {
                    cur++;
                    lst = m;
                    break;
                }
            }
        }
        if(flag)
            inmin(ans,cur);

    }

    cout<<ans;




//    int n,s;
//    cin>>n>>s;
//    vi a(n);
//    for(int i=0;i<n;i++){
//        cin>>a[i];
//    }
//    ll ans=0;
//    for(int i=0;i<n;i++){
//        if(a[i]>s)
//            continue;
//        bitset<N>bt;
//        bt[a[i]]=1;
//        if(a[i]==s){
//            ans=(ans+1LL*(i+1)*(n-i))%MOD;
//        }
//        for(int j=i+1;j<n;j++){
//            if(a[j]>s)
//                continue;
//            if(bt[s-a[j]]==1){
//                cout<<i<<" "<<j<<" "<<(i+1)*(n-j)<<"\n";
//                ans=(ans+1LL*(i+1)*(n-j))%MOD;
//            }
//            bt=(bt|(bt<<a[j]));
//        }
//    }
//
//    cout<<ans;
}


Measured memory_bytes: 384