#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<(n); ++i)
#define rep2(i, s, n) for(int i=s; i<(n); ++i)
#define ALL(v) (v).begin(), (v).end()

using namespace std;

typedef long long ll;

static const int INTINF = (2147483647 >> 1); // 10^9 + 5000
static const ll LLINF = (9223372036854775807 >> 1);
static const int MAX = 1e5+1;
static const ll MOD = 1e9+7;

namespace Kunitaka{
    template<
        typename TYPE,
        std::size_t SIZE
    >
    std::size_t array_length(const TYPE (&array)[SIZE])
    {
        return SIZE;
    }

    template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
    template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
}
using namespace Kunitaka;
namespace Printer{
    void br(){
        cout << endl;
    }

    void loop_cnt(int i){
        cout << "i = " << i << endl;
    }

    void loop_cnt(int i, int j){
        cout << "i = " << i << ", " << "j = " << j << endl;
    }

    template<class T>
    void print(T x){
        cout << x << endl;
    }

    template<class T, class S>
    void print(T x, S y){
        cout << x << "," << y << endl;
    }

    template<class T, class S, class U>
    void print(T x, S y, U z){
        cout << x << "," << y << "," << z << endl;
    }

    template<class T, class S>
    void print(pair<T, S> p){
        cout << p.first << ", " << p.second << endl;
    }

    template<
        typename TYPE,
        std::size_t SIZE
    >
    void print(const TYPE (&array)[SIZE]){
        int lim = array_length(array);
        for(int i=0; i<lim; i++){
            if(i) cout << " ";
            cout << array[i];
        }
        cout << endl;
    }

    template<class S>
    void print(vector<S> v){
        for(int i=0; i<v.size(); i++){
            if(i) cout << " ";
            cout<< v[i];
        }
        cout << endl;
    }

    template<class S>
    void print(vector<vector<S> > vv){
        for(int i=0; i<vv.size(); i++){
            for(int j=0; j<vv[i].size(); j++){
                if(j) cout << " ";
                cout << vv[i][j];
            }
            cout << endl;
        }
    }

    void yesno(bool x){
        if(x) cout << "Yes" << endl;
        else cout << "No" << endl;
    }
};
using namespace Printer;

ll H, W, K; 
string str[11];

ll calc(string out){

    // print(out);

    int ans = 0;

    vector<vector<int> > vv(H, vector<int>(W));
    vector<pair<int, int> > pp;
    int tmp = 0;
    rep(i, out.size()){
        if(out[i] == '1'){
            pp.push_back({tmp, i});
            // print({tmp, i});
            tmp = i+1;
        }
    }


    int kMax =pp.size();
    rep(j, W){
        int k = 0;
        for(pair<int, int> f : pp){
            int s = f.first;
            int e = f.second;
            int sum = 0;
            for(int i=s; i<=e; i++){
                sum += str[i][j] - '0';
            }
            vv[k][j] = sum;
            if(sum > K) return INTINF;
            k++;
        }
    }

    // print(vv);

    vector<int> cnt(kMax);
    rep(j, W){
        rep(i, kMax){
            cnt[i] += vv[i][j];
            if(j != W-1 && cnt[i] + vv[i][j+1] > K){
                rep(t, cnt.size()){
                    cnt[t] = 0;
                }
                ans++;
                break;
            }
        }
    }

    // print(ans + kMax -1);

    return ans + kMax;
}

ll solve(string out){

    if(out.size() == H-1){
        return calc(out+"1");
    }

    ll res = LLINF;

    res =min(res, solve(out + "0"));
    res = min(res, solve(out + "1"));

    return res;
}

int main(int argc, const char * argv[]) {
    //提出時、消す----//
    //--------------//


    // input
    cout << fixed << setprecision(10);
    cin >> H >> W >> K;
    rep(i, H){
        cin >> str[i];
    }
    string tmp = "";
    print(solve(tmp)-1);

    
    
    return 0;
}


Measured memory_bytes: 256