#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rrep(i,n) for(int (i)=((n)-1);(i)>=0;(i)--)
#define itn int
#define miele(v) min_element(v.begin(), v.end())
#define maele(v) max_element(v.begin(), v.end())
#define SUM(v) accumulate(v.begin(), v.end(), 0LL)
#define lb(a, key) lower_bound(a.begin(),a.end(),key)
#define ub(a, key) upper_bound(a.begin(),a.end(),key)
#define COUNT(a, key) count(a.begin(), a.end(), key) 
#define BITCOUNT(x) __builtin_popcount(x)
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
using P = pair <int,int>;
using WeightedGraph = vector<vector <P>>;
using UnWeightedGraph = vector<vector<int>>;
const long long INF = 1LL << 60;
const int MOD = 1000000007;
void YN(bool flg) {cout<<(flg?"YES":"NO")<<'\n';}
void Yn(bool flg) {cout<<(flg?"Yes":"No")<<'\n';}
void yn(bool flg) {cout<<(flg?"yes":"no")<<'\n';}
template <typename T> 
int getIndexOfLowerBound(vector <T> &v, T x){
    return lower_bound(v.begin(),v.end(),x)-v.begin();
}
template <typename T> 
int getIndexOfUpperBound(vector <T> &v, T x){
    return upper_bound(v.begin(),v.end(),x)-v.begin();
}
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
#define DUMPOUT cerr
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
template <typename T, typename U>
istream &operator>>(istream &is, pair<T,U> &p_var) {
    is >> p_var.first >> p_var.second;
    return is;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}
template <typename T>
istream &operator>>(istream &is, vector<vector<T>> &df) {
    for(int i=0;i<df.size();i++)for(int j=0;j<df[i].size();j++)
    is >> df[i][j];
    return is;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
    DUMPOUT<<'{';
    os << pair_var.first;
    DUMPOUT<<',';
    os << " "<< pair_var.second;
    DUMPOUT<<'}';
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &vec) {
    DUMPOUT<<'[';
    for (int i = 0; i < vec.size(); i++) 
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
    DUMPOUT<<']';
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<vector<T>> &df) {
  for (auto& vec : df) os<<vec;
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
    DUMPOUT << "{";
    repi(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) DUMPOUT << ", ";
        itr--;
    }
    DUMPOUT << "}";
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
    DUMPOUT << "{";
    repi(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) DUMPOUT << ", ";
        itr--;
    }
    DUMPOUT << "}";
    return os;
}
void print() {cout << endl;}
template <class Head, class... Tail>
void print(Head&& head, Tail&&... tail) {
  cout << head;
  if (sizeof...(tail) != 0) cout << " ";
  print(forward<Tail>(tail)...);
}
void dump_func() {DUMPOUT << '#'<<endl;}
template <typename Head, typename... Tail>
void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) DUMPOUT << ", ";
    dump_func(std::move(tail)...);
}
#ifdef DEBUG_
#define DEB
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif
vector <string> yokowari;
void bitdfs(int n, string s=""){
    if(s.size()==n){
        yokowari.pb(s);
        return ;
    }
    bitdfs(n, s+"0");
    bitdfs(n, s+"1");
    return ;
}
template <typename T>
struct ruiseki2d{
    T n, m;
    vector <vector<T>> s;
    ruiseki2d(T n,T m):n(n),m(m),s(m+1,vector<T>(n+1)){};
    void add(int y,int x,T w){ 
        s[y+1][x+1]+=w;
    }
    void build(){ 
        rep(i,n)rep(j,m) s[i+1][j+1] += s[i][j+1] + s[i+1][j] - s[i][j];
    }
    T get(int y1, int x1, int y2, int x2){
        if(y1 > y2) swap(y1,y2);
        if(x1 > x2) swap(x1,x2);
        chmin(y1, n); chmax(y1,0LL);
        chmin(y2, n); chmax(y2,0LL);
        chmin(x1, m); chmax(x1,0LL);
        chmin(x2, m); chmax(x2,0LL);
        return s[y2][x2] - s[y1][x2] - s[y2][x1] + s[y1][x1];
    }
};
signed main(void) { cin.tie(0); ios::sync_with_stdio(false);
    int h, w, K; cin>>h>>w>>K;
    vector <vector<char>> g(h,vector <char>(w));
    cin>>g;
    ruiseki2d <int> r(1500, 1500);
    int ans = INF;
    
    rep(i, h)rep(j, w){
        if(g[i][j] == '1') { r.add(i, j, 1); }
    }
    r.build();
    bitdfs(h-1);
    dump(yokowari);
    rep(i, yokowari.size()){
        vector <pair <int,int>> block;
        int tmp = 0;
        int cnt = 0;
        dump(yokowari[i]);
        rep(j, yokowari[i].size()){
            if(yokowari[i][j] == '1'){
                block.pb({tmp, j+1});
                tmp = j + 1;
                cnt++;
            }
        }
        
        block.pb({tmp, h});
        vector <int> num(block.size());
        bool muri = false;
        vector <int> kiri;
        rep(j, w){
            if(muri) break;
            bool rm = false;
            rep(k, block.size()){
                int add = r.get(block[k].F, j, block[k].S, j+1);
                if(add > K) { muri = true; break; }
                num[k] += add;
                if(num[k] > K) rm = true;
            }
            if(rm){
                kiri.pb(j);
                cnt++;
                rep(k, block.size()){
                    int add = r.get(block[k].F, j, block[k].S, j+1);
                    num[k] = add;
                }
            }
        }
        dump(muri, cnt, block, kiri);
        if(!muri) chmin(ans, cnt);
    }
    print(ans);
}


Measured memory_bytes: 17920