#include <bits/stdc++.h>
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
using i64 = int64_t;
using f80 = __float80;
static const int dy[4] = {0,1,0,-1}, dx[4] = {1,0,-1,0};
constexpr i64 INF = 1'010'000'000'000'000'017LL;
constexpr i64 MOD = 1'000'000'007LL;
#define _overload3(_1,_2,_3,name,...) name
#define repi(i,start,end) for(i64 i=(start);i<(end);++i)
#define _rep(i,n) repi(i,0,n)
#define rep(...) _overload3(__VA_ARGS__,repi,_rep,)(__VA_ARGS__)
#define rrep(i,start,end) for(i64 i=(start);i>end;--i)
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define v(T) vector<T>
#define vv(T) v(v(T))
typedef v(i64) vi;
typedef v(vi) vvi;
template<class T> inline bool chmax(T& a,T b){if(a<b){a=b;return true;}return false;}
template<class T> inline bool chmin(T& a,T b){if(a>b){a=b;return true;}return false;}
template<typename T> ostream &operator<<(ostream &os, const vector< T > &v){
  rep(i,v.size()) os << v[i] << (i + 1 != v.size() ? " " : "");
  return os;
}
template<typename T> istream &operator>>(istream &is, vector< T > &v){
  for(T &in : v) is >> in;
  return is;
}

//sort(rng(P),[](const vi &a, const vi &b){return a[1]<b[1];});

i64 pow_mod(i64 n, i64 k, i64 m){
  i64 r = 1;
  for(;k;k>>=1){
    if(k&1) r=(r*n)%m;
    n=(n*n)%m;
  }
  return r;
}

struct uf{
  private:
    vi par, rank, size;
  public:
    uf(i64 n){ init(n); }
    void init(i64 n){
      par.resize(n);
      rank.resize(n,0);
      size.resize(n,1);
      rep(i,n) par[i] = i;
    }
    i64 root(i64 x){
      if(par[x] == x) return x;
      return par[x] = root(par[x]);
    }
    bool same(i64 x, i64 y){ return root(x) == root(y); }
    void unite(i64 x, i64 y){
      x = root(x), y = root(y);
      if(x==y) return;
      if(rank[x]==rank[y]) rank[x]++;
      if(rank[x]<rank[y]) swap(x,y);
      par[y] = x;
      size[x] += size[y];
    }
    i64 num(i64 x){ return size[root(x)]; }
};

void Main()
{
  i64 H,W,K;
  cin >> H >> W >> K;
  vvi S(H, vi(W));
  char c;
  rep(i,H) rep(j,W){
    cin >> c;
    S.at(i).at(j) = c-'0';
  }

  i64 ans = INF;

  rep(i, 1<<(H-1)){
    vi sep;
    rep(j,H-1){
      if(i&(1<<j)) sep.emplace_back(j);
    }
    i64 ans_tmp = sep.size();

    vi count(sep.size()+1,0);

    rep(w,W){
      vi count_n(sep.size()+1,0);
      rep(h,H){
        if(sep.size()==0){
          count_n.at(0) += S.at(h).at(w);
          continue;
        }
        i64 i = 0;
        while(i<sep.size() and h>sep[i]) i++;
        count_n.at(i) += S.at(h).at(w);
      }
      rep(j,sep.size()+1){
        if(count_n.at(j)>K){
          ans_tmp = INF;
          break;
        }
        if(count.at(j) + count_n.at(j) > K){
          ans_tmp += 1;
          rep(k,sep.size()+1){
            count.at(k) = count_n.at(k);
          }
          break;
        }
        count[j] += count_n[j];
      }
      if(ans_tmp >= INF) break;
    }
    chmin(ans, ans_tmp);
  }

  cout << ans << "\n";


}

int main()
{
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  Main();
}

Measured memory_bytes: 384