#include <iostream>
#include <algorithm>
#include <utility>
#include <string>
#include <vector>
#include <set>
#include <stack>
#include <queue>
#include <map>
#include <math.h>
#include <string.h>
#include <iomanip>
#include <numeric>
#include <cstdlib>
#include <cstdint>
#include <cmath>
#include <functional>
#include <limits>
#include <cassert>
#include <bitset>

using namespace std;

/* template */

using ll = long long;

void debug_out() { std::cout << endl; }
 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cout << H << " ";
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cout << "debug: "; debug_out(__VA_ARGS__)
#else
#define debug(...) 
#endif

#define rep(i, a, n) for (int i = (int)(a); i < (int)(n); i++)
#define rrep(i, a, n) for (int i = ((int)(n-1)); i >= (int)(a); i--)
#define Rep(i, a, n) for (long long i = (long long)(a); i< (long long)(n); i++)
#define RRep(i, a, n) for (long long i = ((long long)(n-1ll)); i>=(long long)(a); i--)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()

template <typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> vec) {
  for (std::size_t i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}

struct Edge{
    int to;
    int weight;
    Edge(int t, int w) : to(t), weight(w){ }
};

struct edge{
    int from;
    int to;
    int weight;
    edge(int f,int t,int w) : from(f), to(t), weight(w){ }
};


using Graph = vector<vector<Edge>>;
using graph = vector<vector<int>>;
using edges = vector<edge>;

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;
}

template <class T>
T lcm(T a, T b){
    return (a*b)/gcd(a,b);
}

bool isPalindrome(string s){
    string t = s;
    reverse(all(t));
    return s==t;
}

constexpr ll LNF = __LONG_LONG_MAX__;
constexpr int INF = 1e9+7;
const double PI=3.14159265358979323846;

vector<int> dx = {-1,0,1,0};
vector<int> dy = {0,-1,0,1};

/* template */

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int h,w,k; cin >> h >> w >> k;
    vector<string> s(h);
    rep(i,0,h) cin >> s[i];
    int ans = INF;
    rep(bit,0,(1<<(h-1))){
        vector<int> cut;
        rep(i,0,h-1){
            if(bit & (1<<i)){
                cut.emplace_back(i);
            }
        }
        int t = cut.size();
        vector<int> p(cut.size()+1,0);
        int flag = 0;
        rep(i,0,w){
            int index = 0;
            vector<int> c(cut.size()+1.0);
            rep(j,0,h){
                if(s[j][i]=='1'){
                    c[index]++;
                }
                if(cut.size() && index<cut.size() && j>=cut[index]){
                    index++;
                }
            }
            rep(j,0,cut.size()+1){
                if(c[j]>k){
                    flag = 1;
                    break;
                }
                p[j] += c[j];
                if(p[j]>k){
                    rep(q,0,cut.size()+1){
                        p[q] = c[q];
                    }   
                    t++;
                    break;
                }
            }
            if(flag) break;
        }
        if(flag) continue;
        chmin(ans,t);
    }
    cout << ans << endl;
}                               

Measured memory_bytes: 256