#define _USE_MATH_DEFINES
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <clocale>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <regex>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

#define IOS ios::sync_with_stdio(false); cin.tie(0);
#define FOR(i, s, n) for(int i = (s), i##_len=(n); i < i##_len; ++i)
#define FORS(i, s, n) for(int i = (s), i##_len=(n); i <= i##_len; ++i)
#define VFOR(i, s, n) for(int i = (s); i < (n); ++i)
#define VFORS(i, s, n) for(int i = (s); i <= (n); ++i)
#define REP(i, n) FOR(i, 0, n)
#define REPS(i, n) FORS(i, 0, n)
#define VREP(i, n) VFOR(i, 0, n)
#define VREPS(i, n) VFORS(i, 0, n)
#define RFOR(i, s, n) for(int i = (s), i##_len=(n); i >= i##_len; --i)
#define RFORS(i, s, n) for(int i = (s), i##_len=(n); i > i##_len; --i)
#define RREP(i, n) RFOR(i, n, 0)
#define RREPS(i, n) RFORS(i, n, 0)
#define ALL(v) (v).begin(), (v).end()
#define SORT(v) sort(ALL(v))
#define RSORT(v) sort(ALL(v), greater<decltype(v[0])>())
#define SZ(x) ((int)(x).size())
#define REV(x) reverse(ALL(x))
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(ALL(v)), v.end())

using ld = long double;
using ll = long long;
using ui = unsigned int;
using ull = unsigned long long;
using Pi_i = pair<int, int>;
using Pll_ll = pair<ll, ll>;
using VB = vector<bool>;
using VC = vector<char>;
using VD = vector<double>;
using VI = vector<int>;
using VLL = vector<ll>;
using VS = vector<string>;
using VSH = vector<short>;
using VULL = vector<ull>;

const int MOD = 1000000007;
const int INF = 1000000000;
const int NIL = -1;
const double EPS = 1E-10;

template<class T, class S>
bool chmax(T &a, const S &b){
    if(a < b){
        a = b; return true;
    }
    return false;
}
template<class T, class S>
bool chmin(T &a, const S &b){
    if(b < a){
        a = b; return true;
    }
    return false;
}


int H, W, K;
vector<VI> num;

inline int rectNum(int a, int b, int c, int d){
    return num[c][d] - num[a-1][d] - num[c][b-1] + num[a-1][b-1];
}

int main(){
    cin >> H >> W >> K;
    num.resize(H+1);
    num[0].resize(W+1, 0);
    REP(i, H){
        string s; cin >> s;
        num[i+1].resize(W+1, 0);
        REP(j, W){
            num[i+1][j+1] = s[j] - '0' + num[i][j+1] + num[i+1][j] - num[i][j];
        }
    }
  /*  REP(i, H){
        REP(j, W){
            if(j) cout << " ";
            cout << num[i+1][j+1];
        }
        cout << "\n";
    }*/
    if(num[H][W] <= K){
        cout << 0 << endl;
        return 0;
    }
    int ans(1010110);
    REP(i, 1<<(H-1)){
        VI h;
        bool impos = false;
        h.PB(0);
        for(int j = 0; j < H-1; ++j)
            if(i & (1<<j)) h.EB(j+1);
        int n(h.size());
        h.PB(H);
        int prv(0), cnt(n-1);
        FORS(j, 1, W){
            bool okay = true;
            REP(k, n){
                if(rectNum(h[k]+1, prv+1, h[k+1], j) > K){
                    okay = false; break;
                }
            }
            if(!okay){
                if(prv + 1 == j) impos = true;
                ++cnt; prv = j-1; --j;
            }
            if(impos) break;
        }
        if(impos) continue;
        chmin(ans, cnt);
    }
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 256