#include <bits/stdc++.h>
using namespace std;

// clang-format off
#define  range(i, l, r) for (int i = (int)(l); i < (int)(r); ++(i))
#define rrange(i, l, r) for (int i = (int)(r) - 1; i >= (int)(l); --(i))

#define  whole(f, x, ...) ([&](decltype((x)) container) { return (f)(  begin(container),  end(container), ## __VA_ARGS__); })(x)
#define rwhole(f, x, ...) ([&](decltype((x)) container) { return (f)( rbegin(container), rend(container), ## __VA_ARGS__); })(x)

#define endl '\n'
#define debug(x) cerr << "(" << __LINE__ << ")" << #x << ": " << (x) << endl

using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;

constexpr i32 mod   = 1e9 + 7;
constexpr i32 inf   = 1001001001;
constexpr i64 infll = 1001001001001001001ll;

constexpr int dx[] = {0, -1, 1, 0, -1, 1, -1, 1}; 
constexpr int dy[] = {-1, 0, 0, 1, -1, -1, 1, 1};

struct IoSetup { IoSetup(int x = 15){ cin.tie(0); ios::sync_with_stdio(0); cout << fixed << setprecision(x); cerr << fixed << setprecision(x); } } iosetup;

template <typename T = i64> T input() { T x; cin >> x; return x; }

template <typename T> ostream &operator<<(ostream &os, vector<T> &v) { range(i, 0, v.size()) { os << v[i] << (i + 1 != v.size() ? " " : ""); } return os; } 
template <typename T> istream &operator>>(istream &is, vector<T> &v) { for (T &in : v) is >> in; return is; }

template <typename T1, typename T2> ostream &operator<<(ostream &os, pair<T1, T2> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
template <typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }

template <typename T> vector<T> make_vector(size_t a, T b) { return vector<T>(a, b); }
template <typename... Ts> auto make_vector(size_t a, Ts... ts) { return vector<decltype(make_vector(ts...))>(a, make_vector(ts...)); }

template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }
// clang-format on

void solver() {
    int H = input(), W = input(), K = input();
    vector< string > s(H);
    cin >> s;
    
    int ans = inf;
    range(bit, 0, 1 << H) {
        if (bit & 1) continue;
//        debug(bit);
//        range(i, 0, H) {
//            if (bit & (1 << i)) cout << endl;
//            cout << s[i] << endl;
//        }
        
        int cnt = 0;
        range(i, 0, H) {
            if (bit & (1 << i)) cnt++;
        }
        
        vector< int > t(cnt + 1);
        
        int res = cnt;
        range(j, 0, W) {
            int idx = 0;
            range(i, 0, H) {
                if (bit & (1 << i)) idx++;
                t[idx] += s[i][j] == '1';
            }
            
//            debug(t);
            if (*whole(max_element, t) > K) {
                res++;
                t = make_vector(cnt + 1, 0);
//                debug(t.size());
                idx = 0;
                range(i, 0, H) {
                    if (bit & (1 << i)) idx++;
                    t[idx] += s[i][j] == '1';
                }

                if (*whole(max_element, t) > K) res = inf;
            }
        }
//        debug(res);
        
        chmin(ans, res);
    }
    
    cout << ans << endl;
}

signed main(int argc, char *argv[]) {
    solver();
}


Measured memory_bytes: 256