#include <bits/stdc++.h>
#define ALL(v) std::begin(v), std::end(v)
#define ALLR(v) std::rbegin(v), std::rend(v)
using ll = std::int64_t;
using ull = std::uint64_t;
using pii = std::pair<int, int>;
using tii = std::tuple<int, int, int>;
using pll = std::pair<ll, ll>;
using tll = std::tuple<ll, ll, ll>;
template <typename T> using vec = std::vector<T>;
template <typename T> using vvec = vec<vec<T>>;
template <typename T> const T& var_min(const T &t) { return t; }
template <typename T> const T& var_max(const T &t) { return t; }
template <typename T, typename... Tail> const T& var_min(const T &t, const Tail&... tail) { return std::min(t, var_min(tail...)); }
template <typename T, typename... Tail> const T& var_max(const T &t, const Tail&... tail) { return std::max(t, var_max(tail...)); }
template <typename T, typename... Tail> void chmin(T &t, const Tail&... tail) { t = var_min(t, tail...); }
template <typename T, typename... Tail> void chmax(T &t, const Tail&... tail) { t = var_max(t, tail...); }
template <typename T> const T& clamp(const T &t, const T &low, const T &high) { return std::max(low, std::min(high, t)); }
template <typename T> void chclamp(T &t, const T &low, const T &high) { return t = clamp(t, low, high); }
template <typename T> T make_v(T init) { return init; }
template <typename T, typename... Tail> auto make_v(T init, std::size_t s, Tail... tail) { auto v = std::move(make_v(init, tail...)); return vec<decltype(v)>(s, v); }
template <typename T, std::size_t Head, std::size_t ...Tail> struct multi_dem_array { using type = std::array<typename multi_dem_array<T, Tail...>::type, Head>; };
template <typename T, std::size_t Head> struct multi_dem_array<T, Head> { using type = std::array<T, Head>; };
template <typename T, std::size_t ...Args> using mdarray = typename multi_dem_array<T, Args...>::type;
namespace init__ { struct InitIO { InitIO() { std::cin.tie(nullptr); std::ios_base::sync_with_stdio(false); std::cout << std::fixed << std::setprecision(30); } } init_io; }

int main() {
    ll H, W, K;
    std::cin >> H >> W >> K;
    vec<std::string> vs(H);
    for (auto &e : vs) std::cin >> e;

    ll ans = H * W + 1;
    for (ll s = 0; s < (1ll << (H - 1)); s++) {
        vec<ll> split;
        split.push_back(0);
        bool imp = false;
        for (ll i = 0; i < H - 1; i++) if ((s >> i) & 1) split.push_back(i + 1);
        split.push_back(H);
        vec<ll> cnts(split.size() - 1);
        vec<ll> col(split.size() - 1);
        ll res = std::bitset<32>(s).count();
        for (ll w = 0; w < W; w++) {
            std::fill(ALL(col), 0);
            for (ll i = 0; i + 1 < split.size(); i++) {
                ll f = split[i], t = split[i + 1];
                for (ll j = f; j < t; j++) if (vs[j][w] == '1') col[i]++;
            }
            bool ok = true;
            for (ll i = 0; i < col.size(); i++) {
                if (K < col[i]) {
                    imp = true;
                    break;
                }
                if (K < cnts[i] + col[i]) {
                    ok = false;
                    break;
                }
            }
            if (imp) break;
            if (!ok) {
                std::fill(ALL(cnts), 0);
                res++;
            }
            for (ll i = 0; i < col.size(); i++) cnts[i] += col[i];
        }
        if (!imp) chmin(ans, res);
    }

    std::cout << ans << '\n';
    return 0;
}


Measured memory_bytes: 256