#include <bits/stdc++.h>
 
using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl;
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
typedef long long ll;
#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
#define Sp(p) cout<<setprecision(25)<< fixed<<p<<endl;
//int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
//int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
vi dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
vi dx2 = { 1,1,0,-1,-1,-1,0,1 }, dy2 = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
//const ll MOD = 1000000007;
const ll MOD = 998244353;
// #define mp make_pair
//#define endl '\n'


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

    vii a(h, vi(w));
    rep (i, h) {
        rep (j, w) {
            char c;
            cin >> c;
            a[i][j] = c - '0';
        }
    }
    DEBUG_MAT(a);

    vii sum(h + 1, vi(w + 1));
    rep1 (i, h) {
        rep (j, w) {
            sum[i][j + 1] = sum[i][j] + a[i - 1][j];
        }
    }
    rep (i, h) {
        rep (j, w + 1) {
            sum[i + 1][j] += sum[i][j];
        }
    }
    DEBUG_MAT(sum);

    int ans = inf;
    rep (i, 1 << (h - 1)) {
        vi div(1, 0);
        int cnt = 0;
        rep (j, h - 1) {
            if (i & (1 << j)) {
                div.push_back(j + 1);
                cnt++;
            }
        }
        div.push_back(h);
        //DEBUG_VEC(div);

        int pre = 0;
        for (int j = 0; j < w; j++) {
            bool flag = true;
            rep (i, div.size() - 1) {
                int y0 = div[i], y1 = div[i + 1];
                int x0 = pre, x1 = j + 1;
                if (sum[y1][x1] - sum[y1][x0] - sum[y0][x1] + sum[y0][x0] > k) {
                    flag = false;
                }
            }
            if (not flag) {
                if (pre == j) {
                    cnt = inf;
                    break;
                }
                else {
                    cnt++;
                    pre = j;
                    j--;
                }
            }
        }
        //DEBUG(cnt);
        chmin(ans, cnt);
    }
    cout << ans << endl;

}

Measured memory_bytes: 384