#include <bits/stdc++.h>
using namespace std;



typedef unsigned long ul;
typedef unsigned long long ull;
typedef long long ll;
typedef vector<ll> vint;
typedef vector< vector<ll> > vvint;
typedef vector< vector< vector<ll> > > vvvint;
typedef vector<string> vstring;
typedef vector< vector<string> > vvstring;
typedef vector<char> vchar;
typedef vector< vector<char> > vvchar;
typedef vector<long double> vdouble;
typedef vector< vector<long double> > vvdouble;
typedef vector< vector< vector<long double> > > vvvdouble;
typedef pair<ll,ll> pint;
typedef vector<pint> vpint;
typedef vector<bool> vbool;

#define rep(i,n) for(ll i=0;i<n;i++)
#define repf(i,f,n) for(ll i=f;i<n;i++)
#define repr(i,n) for(ll i=n-1;i>=0;i--)
#define mp make_pair
#define mt make_tuple
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define ALL(obj) (obj).begin(), (obj).end()
// #define LLONG_MAX 9223372036854775806
#define vmax(vec) *max_element(vec.begin(), vec.end())
#define vmin(vec) *min_element(vec.begin(), vec.end())
#define vsort(vec) sort(vec.begin(), vec.end())
#define vsortgr(vec) sort(vec.begin(), vec.end(), greater<ll>())
#define MOD 1000000007
// #define MOD 998244353
// #define MOD LLONG_MAX
const double PI=3.14159265358979323846;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }




int dy[]={0, 0, 1, -1};
int dx[]={1, -1, 0, 0,};



void printv(vint &v){
  for(auto e:v) cout<<e<<" ";
  cout<<endl;
}


// 繰り返し二乗法
ll power(ll a, ll b){
  if(a==1)return a;
  // if(a==0)re
  ll res=1;
  while(b>0){
    if(b&1) res=res*a%MOD;
    a=a*a%MOD;
    b>>=1;
  }
  return res;
}

const int MAX=2100000;
 
ll fact[MAX], fact_inv[MAX];
 
void init_fact(ll n){
  fact[0]=1;
  // 階乗の計算
  rep(i, n) fact[i+1]=fact[i]*(i+1)%MOD;
  fact_inv[n]=power(fact[n], MOD-2);
  // 逆元の計算
  for(ll i=n-1;i>=0;i--) fact_inv[i]=fact_inv[i+1]*(i+1)%MOD;
}
 
ll comb(ll n, ll r){
  return (fact[n]*fact_inv[r])%MOD*fact_inv[n-r]%MOD;
}
 
ll perm(ll n, ll r){
  return (fact[n]*fact_inv[n-r])%MOD;
}






int main() {
  cout<<fixed<<setprecision(10);


  ll h,w,k; cin>>h>>w>>k;
  vstring s(h); rep(i,h) cin>>s[i];

  auto f = [&](vvint &v){
    // puts("hoge");
    ll rtn = 0;
    ll x, y;
    y = v.size();
    x = v[0].size();

    vint cnt(v.size(), 0);
    rep(i, x){
      ll maxval = 0;
      ll par = 0;
      rep(j,y){
        cnt[j] += v[j][i];
        chmax(par, v[j][i]);
        chmax(maxval, cnt[j]);
      }
      if(par > k){
        return 100000ll;
      }
      if(maxval > k){
        rtn++;
        rep(j,y) cnt[j] = v[j][i];
      }
    }
    return rtn;
  };

  if(h == 1){
    vvint v(1, vint(w, 0));
    rep(i, w){
      if(s[0][i] == '1') v[0][i] = 1;
    }
    // cout << f(v) <<endl;
    return 0;
  }
  ll ans = LLONG_MAX;
  rep(i, 1<<(h-1)){
    vvint v(1, vint(w, 0));
    rep(ii, w){
      if(s[0][ii] == '1') v.back()[ii]++;
    }
    ll tmp = 0;
    rep(j, h-1){
      if(i&(1<<j)){
        v.push_back(vint(w, 0));
        tmp++;
      }
      rep(ii, w){
        if(s[j+1][ii] == '1') v.back()[ii]++;
      }
    }
    // cout<<tmp<<" "<<f(v)<<endl;
    chmin(ans, tmp+f(v));
  }
  cout<<ans<<endl;

}






Measured memory_bytes: 384