#include <algorithm>
#include <complex>
#include <cstdlib>
#include <ctime>
#include <time.h>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>

using namespace std;
#define SAY_YES cout << "YES" << endl;
#define SAY_Yes cout << "Yes" << endl;
#define SAY_NO cout << "NO" << endl;
#define SAY_No cout << "No" << endl;
#define IFYES(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "YES" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "NO" << endl;  \
    }
#define IFYes(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "Yes" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "No" << endl;  \
    }
#define IFyes(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "yes" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "no" << endl;  \
    }
#define DEBUG_OUTPUT_ARRAY(XXX, ONE)                            \
    for (int i = 0; i < (ONE); i++)                             \
    {                                                           \
        cout << "DEBUG: i = " << i << " -> " << XXX[i] << endl; \
    }
#define DEBUG_OUTPUT_ARRAY2(XXX, ONE, TWO)                             \
    for (int i = 0; i < (ONE); i++)                                    \
    {                                                                  \
        cout << "<<< i = " << i << " >>>" << endl;                     \
        for (int j = 0; j < (TWO); j++)                                \
        {                                                              \
            cout << "DEBUG: j = " << j << " -> " << XXX[i][j] << endl; \
        }                                                              \
    }
#define DEBUG_OUTPUT_ARRAY2_BOX(XXX, ONE, TWO) \
    for (int i = 0; i < (ONE); i++)            \
    {                                          \
        cout << i << "  ";                     \
        for (int j = 0; j < (TWO); j++)        \
        {                                      \
            cout << XXX[i][j] << " ";          \
        }                                      \
        cout << endl;                          \
    }

typedef pair<long long int, long long int> pll;


const long long int mod = 1000000007;
const long long int INF = 1e18;




long long int H,W,K,res=INF;
long double L;
string S[20];

int main()
{
    cout << fixed << setprecision(18);
    cin>>H>>W>>K;
    for(long long int i = 0; i < H; i++){
        cin>>S[i];
    }
    long long int white[15]={},newwhite[15]={};
    for(long long int i = 0; i < (1<<(H-1)); i++){
        bool ok=true;
        long long int cou=0;

        for(long long int i = 0; i < H; i++){
            white[i]=0;
            newwhite[i]=0;
        }
        for(long long int j=0;j<W;j++){
            for(long long int k = 0; k < H; k++){
                newwhite[k]=0;
            }
            for(long long int k = 0; k < H; k++){
                if(S[k][j]=='1'){white[k]++;newwhite[k]++;}
            }

            //壊すかの判定
            long long int whitenum=0;
            bool crush=false;
            whitenum=white[0];
            for(long long int k = 1; k < H; k++){
                if((i & (1<<(k-1)))!=0){
                    if(whitenum>K){crush=true;}
                    whitenum=white[k];
                }else{
                    whitenum+=white[k];
                    if(whitenum>K){crush=true;}
                }

            }
            
            if(whitenum>K){crush=true;}

            if(crush&&j==0){ok=false;}
            if(crush){
                cou++;
                whitenum=newwhite[0];
                for(long long int k = 1; k < H; k++){
                    if((i & (1<<(k-1)))!=0){
                        if(whitenum>K){ok=false;}
                        whitenum=newwhite[k];
                    }else{
                        whitenum+=newwhite[k];
                        if(whitenum>K){ok=false;}
                    }

                }

                for(long long int k = 0; k < H; k++){
                    white[k]=newwhite[k];
                }
            }
        }
        for(long long int j = 0; j < H-1; j++){
            if(((1<<j)&i)!=0){
                cou++;
            }
        }
        if(ok){res=min(res,cou);}
    }
    cout<<res<<endl;
}


Measured memory_bytes: 256