#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
using VB = vector<bool>;
using VVI = vector<VI>;
using VVL = vector<VL>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)
#define rep3(i, s, n, d) for (int i = (s); i < (int)(n); i += (d))
#define allpt(v) (v).begin(), (v).end()
#define allpt_c(v) (v).cbegin(), (v).cend()
#define allpt_r(v) (v).rbegin(), (v).rend()

const int mod1 = 1e9 + 7, mod2 = 998244353, mod3 = 1e9 + 9;
const int mod = mod1;
const ll inf = 1e18;

const string wsp = " ";
const string tb = "\t";
const string rt = "\n";

template <typename T>
void show1dvec(const vector<T> &v)
{
    if (v.size() == 0)
        return;
    int n = v.size() - 1;
    rep(i, n) cout << v[i] << wsp;
    cout << v[n] << rt;

    return;
}

template <typename T>
void show2dvec(const vector<vector<T>> &v)
{
    int n = v.size();
    rep(i, n) show1dvec(v[i]);
}

template <typename T, typename S>
void show1dpair(const vector<pair<T, S>> &v)
{
    int n = v.size();
    rep(i, n) cout << v[i].first << wsp << v[i].second << rt;
    return;
}

template <typename T, typename S>
void pairzip(const vector<pair<T, S>> &v, vector<T> &t, vector<T> &s)
{
    int n = v.size();
    rep(i, n)
    {
        t.push_back(v[i].first);
        s.push_back(v[i].second);
    }
    return;
}

template <typename T>
void maxvec(vector<T> &v)
{
    T s = v[0];
    int n = v.size();
    rep(i, n - 1)
    {
        if (s > v[i + 1])
        {
            v[i + 1] = s;
        }
        s = v[i + 1];
    }
}

template <typename T, typename S>
bool myfind(T t, S s)
{
    return find(t.cbegin(), t.cend(), s) != t.cend();
}

bool check(int y, int x, int h, int w)
{
    return 0 <= y && y < h && 0 <= x && x < w;
}

template <typename T>
vector<T> cumsum(const vector<T> &v)
{
    T s = 0;
    vector<T> ret;

    rep(i, v.size())
    {
        s += v[i];
        s %= mod;
        ret.emplace_back(s);
    }
    return ret;
}

bool iskadomatsu(int a, int b, int c)
{
    return (a != b && b != c && c != a) && ((a > b && b < c) || (a < b && b > c));
}

VS split(string s, char c)
{
    VS ret;
    string part;
    s += c;
    rep(i, s.length())
    {
        if (s[i] == c)
        {
            ret.emplace_back(part);
            part = "";
        }
        else if (s[i] != c)
        {
            part += s[i];
        }
    }
    return ret;
}

string removezero(string &s)
{
    string ret;
    for (auto z : s)
        if (z != '0')
            ret += z;
    return ret;
}

ll sumdigit(ll x)
{
    ll ans{0};
    while (x > 0)
    {
        ans += x % 10;
        x /= 10;
    }
    return ans;
}

template <typename T, typename S, typename R>
ll pow_mod(T p, S q, R mod = 1ll)
{
    ll ret = 1, r = p;
    while (q)
    {
        if (q % 2)
            ret *= r, ret %= mod;
        r = (r * r) % mod, q /= 2;
    }
    return ret % mod;
}

ll pow_mod(int p, int q, int mod)
{
    ll ret = 1, r = p;
    while (q)
    {
        if (q % 2)
            ret *= r, ret %= mod;
        r = (r * r) % mod, q /= 2;
    }
    return ret % mod;
}

void make_frac_tables(int n, VL &frac_list, VL &frac_inv_list)
{
    rep(i, n)
    {
        frac_list[i + 1] *= frac_list[i] * (i + 1);
        frac_list[i + 1] %= mod;
        frac_inv_list[i + 1] *= frac_inv_list[i] * pow_mod(i + 1, mod - 2, mod);
        frac_inv_list[i + 1] %= mod;
    }
}

ll comb(int a, int b, const VL &frac_list, const VL &frac_inv_list)
{
    if (a < b)
        return 0;
    ll ret = frac_list[a];
    ret *= frac_inv_list[b];
    ret %= mod;
    ret *= frac_inv_list[a - b];
    ret %= mod;
    return ret;
}

int cut_choco(int x, const int h, const int w, const int k, const vector<VB> &is_whitechoco)
{
    int y{1}, z, c{0}, ret{h * w};
    VB is_cut_row;
    bool ng;

    rep(i, h - 1)
    {
        is_cut_row.emplace_back(x % 2);
        x /= 2;
    }
    y += count(allpt_c(is_cut_row), 1);

    VVI choco(y, VI(w));
    VI greedy_sum(y, 0);


    z = 0;
    ng = false;
    rep(i, h)
    {
        rep(j, w)
        {
            choco[z][j] += is_whitechoco[i][j];
            if (choco[z][j] > k)
            {
                ng = true;
                break;
            }
        }
        if (i != h - 1 && is_cut_row[i])
            z++;
        if (ng)
            return ret;
    }
    // show1dvec(is_cut_row);
    // show2dvec(choco);

    rep(j, w)
    {
        rep(i, y) greedy_sum[i] += choco[i][j];
        if (*max_element(allpt_c(greedy_sum)) > k)
        {
            fill(allpt(greedy_sum), 0);
            c++;
            rep(i, y) greedy_sum[i] += choco[i][j];
        }
    }
    ret = c + y - 1;

    return ret;
}


int main()
{

    cin.tie(0);
    ios::sync_with_stdio(false);

#ifdef DEBUG
    cout << "DEBUG MODE" << endl;
    ifstream in("input.txt"); //for debug
    cin.rdbuf(in.rdbuf());    //for debug
#endif

    int h, w, k, ans;
    cin >> h >> w >> k;
    ans = h * w;
    string s;
    vector<VB> is_whitechoco(h, VB(w));
    rep(i, h)
    {
        cin >> s;
        rep(j, w)
        {
            is_whitechoco[i][j] = (s[j] == '1');
        }
    }

    rep(i, 1 << h - 1)
        ans = min(ans, cut_choco(i, h, w, k, is_whitechoco));
    cout << ans << rt;

    return 0;
}


Measured memory_bytes: 256