//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <fstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
#include <limits>
 
using namespace std;
 
//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
 
//math
//-------------------------------------------
template<class T> inline T sqr(T x) { return x * x; }
 
//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
 
//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define EXISTch(s,c) ((((s).find_first_of(c)) != std::string::npos)? 1 : 0)//cがあれば1 if(1)
#define SORT(c) sort((c).begin(),(c).end())
 
#define REP(i,n) for(int i=0;i<(int)n;++i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
 
//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int INF = (int)1000000007;
const LL MOD = (LL)1000000007;//10^9+7
const LL INF2 = (LL)100000000000000000;//10^18


int main() {
    LL H,W,K;
    cin>>H>>W>>K;
    VVI S(H);
    REP(i,H){
        REP(j,W){
            char s;
            cin>>s;
            S[i].PB(s - '0');
        }
    }

    int ans = INF;
    for(unsigned int bit = 0;bit<(1<<(H-1));bit++){
        // cout<<"-----------bit :"<<bit<<endl;
        int split = __builtin_popcount(bit);
        int count = split;
        bool flag = false;
        VI white(split+1, 0);
        REP(w,W){
            int sp = 0;
            bool check = false;
            REP(h,H){
                white[sp] += S[h][w];

                if(white[sp]>K){
                    check = true;
                    break;
                }
                if(bit&(1<<h)){
                    sp++;
                }
            }
            if(check){
                count++;
                white.assign(split+1,0);
                sp = 0;
                REP(h,H){
                    white[sp] += S[h][w];

                    if(white[sp]>K){
                        flag = true;
                        break;
                    }
                    if(bit&(1<<h)){
                        sp++;
                    }
                }
                if(flag){
                    break;
                }
            }
            
        }

        if(!flag){
            ans = (count<=ans) ? count: ans;
        }
        

    }
    cout<<ans<<endl;
}


Measured memory_bytes: 256