#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define countof(array) (sizeof(array) / sizeof(array[0]))
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = (n)-1; i >= 0; --i)
#define rep2(i,n) for(int i = 1; i <= (n); ++i)
#define rrep2(i,n) for(int i = (n); i > 0; --i)
#define srep(i,s,n) for(int i = s; i < (n); ++i)
#define rsrep(i,s,n) for(int i = (n)-1; i >= s; --i)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define aall(a) (a), (a)+countof(a)//for array sorting
#define raall(a) (a), (a)+countof(a), greater<>()
#define show(x) cout<<#x<<" = "<<(x)<<endl;
#define vfind(v, a) find(all(v), a) != v.end()
#define yn(f) { if (f) puts("YES"); else puts("NO"); }
#define yns(f) { if (f) puts("Yes"); else puts("No"); }
#define show_ary(...) { cout<<#__VA_ARGS__<<" = "; for (const auto& x : (__VA_ARGS__)) { cout<<x<<" "; } cout<<endl; }
#define show_pair(...) cout<<#__VA_ARGS__<<" = "<<endl; for (const auto& x : (__VA_ARGS__)) { cout<<"  "<<x.fi<<" : "<<x.se<<endl; }
#define out_ary(...) { int n = (__VA_ARGS__).size(); rep(i, n) printf("%d%s", (__VA_ARGS__)[i], i != n-1 ? " " : "\n"); }
#define argmax(v) distance((v).begin(), max_element(all(v)))
#define argmin(v) distance((v).begin(), min_element(all(v)))
#define vmax(v) *max_element(all(v))
#define vmin(v) *min_element(all(v))
typedef long long int ll;
typedef pair<int, int> P;
typedef vector<P> vpair;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<double> vdouble;
typedef vector<string> vstr;
typedef vector<bool> vbool;
typedef vector<vint> vvint;
typedef vector<vll> vvll;
typedef vector<vstr> vvstr;
typedef vector<vbool> vvbool;
const ll LINF = 2000000000000000000ll;
const int INF = 1000000100;
const ll MOD = 1e9+7;

struct CumulativeSum2D {
    int h, w;
    vvll data;

    CumulativeSum2D(int H, int W) {
        h = H;
        w = W;
        data.resize(h+1, vll(w+1, 0));
    } 

    void add(int x, int y, ll z) {
        if(x >= h || y >= w) return;
        ++x, ++y;
        data[x][y] += z;
    }

    void build() {
        for(int i = 1; i <= h; i++) {
            for(int j = 1; j <= w; j++) {
                data[i][j] += data[i][j - 1] + data[i - 1][j] - data[i - 1][j - 1];
            }
        }
    }

    ll query(int sx, int sy, int gx, int gy) {
        return (data[gx][gy] - data[sx][gy] - data[gx][sy] + data[sx][sy]);
    }
};

int main() {
    int h, w, k;
    cin >> h >> w >> k;
    vstr s(h);
    rep(i, h) cin >> s[i];

    CumulativeSum2D sum(h, w);
    rep(i, h) rep(j, w) if (s[i][j] == '1') sum.add(i, j, 1);
    sum.build();

    int ans = INF;
    rep(id, pow(2, h-1)) {
        vint ls;
        int num = id;
        int count = 1;
        while(num) {
            if (num&1) ls.push_back(count);
            num >>= 1; 
            count++;
        }
        ls.push_back(h);
        int t = 0;
        int tmp = ls.size()-1;
        rep2(i, w) {
            int l = 0;
            rep(j, ls.size()) {
                int white = sum.query(l, t, ls[j], i);
                // cout << l << " " << t << " " << ls[j] << " " << i << endl;
                // show(white);
                if (white > k) {
                    if (i-t == 1) tmp = INF;
                    tmp++;
                    t = i-1;
                    break;
                }
                l = ls[j];
            }
        }
        ans = min(ans, tmp);
    }
    


    cout << ans << endl;
    return 0;
}

Measured memory_bytes: 384