#include <vector>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <numeric>
#include <sstream>
#include <iostream>
#include <iomanip>
#define _USE_MATH_DEFINES
#include <cmath>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <cfloat>
#include <cmath>
#include <ctime>
#include <cassert>
#include <cctype>
#include <cstdio>
#include <cassert>
using namespace std;

#define EPS 1e-12
#define ull unsigned long long
#define ll long long
#define VI vector<ll>
#define PII pair<ll, ll> 
#define VVI vector<vector<ll> >
#define VVVI vector<vector<vector<ll>>>
#define VVVVI vector<vector<vector<vector<ll>>>>
#define REP(i,n) for(ll i=0,_n=(n);(i)<(ll)_n;++i)
#define REPR(i,n) for(ll i=(ll)(n)-1;0<=(i);--i)
#define RANGE(i,a,b) for(ll i=(ll)a,_b=(ll)(b);(i)<_b;++i)
#define RANGER(i,a,b) for(ll i=(ll)(b)-1,_a=(ll)(a);(_a)<=i;--i)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define ALL(c) (c).begin(), (c).end()
#define ALLR(c) (c).rbegin(), (c).rend()
#define PB push_back
#define MP(a, b) make_pair(a, b)
#define POPCOUNT __builtin_popcount
#define POPCOUNTLL __builtin_popcountll
#define CLEAR(table, v) memset(table, v, sizeof(table));
#define UNIFORM_DOUBLE(a, b) (((b-a)*(double)rand()/RAND_MAX)+a) // [a, b) 
#define UNIFORM_LL(a, b) (ll)UNIFORM_DOUBLE(a, b) // [a, b) 
#define IN(v, lo, hi) ((lo)<=(v) && (v)<(hi))
#define DD(v) cout<<#v<<": "<<v<<endl
#define FI first
#define SE second
template <typename T0, typename T1> std::ostream& operator<<(std::ostream& os, const map<T0, T1>& v) { for( typename map<T0, T1>::const_iterator p = v.begin(); p!=v.end(); p++ ){os << p->first << ": " << p->second << " ";} return os; }
template <typename T0, typename T1> std::ostream& operator<<(std::ostream& os, const pair<T0, T1>& v) { os << v.first << ": " << v.second << " "; return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const vector<T>& v) { for( int i = 0; i < (int)v.size(); i++ ) { os << v[i] << " "; } return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const set<T>& v) { vector<T> tmp(v.begin(), v.end()); os << tmp; return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const deque<T>& v) { vector<T> tmp(v.begin(), v.end()); os << tmp; return os; }
template <typename T> std::ostream& operator<<(std::ostream& os, const vector<vector<T> >& v) { for( int i = 0; i < (int)v.size(); i++ ) { os << v[i] << endl; } return os; }
template<typename T>void maxUpdate(T& a, T b) {a = max(a, b);}
template<typename T>void minUpdate(T& a, T b) {a = min(a, b);}

#define MOD 1000000007LL
#define INF (1LL<<60)

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	ll K,W,H;
	string s;
	while(cin>>H>>W>>K) {
		VVI m(H, VI(W));
		REP(y, H) {
			cin>>s;
			REP(x, W) m[y][x] = s[x]-'0';
		}
		ll ans = 1LL<<60;
		REP(bi, 1<<(H-1)) {
			VI seq;
			seq.PB(1);
			REP(y, H-1) {
				if(bi>>y&1) seq[seq.size()-1]++;
				else seq.PB(1);
			}
			VVI w(seq.size(), VI(W));
			ll sy=0;
			REP(i, seq.size()) {
				REP(x, W) RANGE(y, sy, sy+seq[i]) w[i][x]+=m[y][x];
				sy+=seq[i];
			}
			bool ok = true;
			ll lans = seq.size()-1;
			VI sum(seq.size());
			REP(x, W) {
				REP(y, w.size()) {
					if(K<w[y][x]) ok=false;
					sum[y]+=w[y][x];
				}
				bool cut = false;
				REP(y, w.size()) {
					if(K<sum[y]) cut = true;
				}
				if(cut) {
					lans++;
					REP(y, w.size()) {
						sum[y] = w[y][x];
					}
				}
			}
//			DD(bi);
//			DD(w);
//			DD(seq);
//			DD(seq.size());
//			DD(lans);
			if(ok) minUpdate(ans, lans);
		}
		cout<<ans<<endl;
	}
	
	return 0;
}


Measured memory_bytes: 1016