//#pragma GCC optimize("Ofast,no-stack-protector")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#pragma GCC optimize("unroll-loops")
/*#pragma GCC optimize("section-anchors")
#pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
#pragma GCC optimize("vpt")
#pragma GCC optimize("rename-registers")
#pragma GCC optimize("move-loop-invariants")
#pragma GCC optimize("unswitch-loops")
#pragma GCC optimize("function-sections")
#pragma GCC optimize("data-sections")
#pragma GCC optimize("branch-target-load-optimize")
#pragma GCC optimize("branch-target-load-optimize2")
#pragma GCC optimize("btr-bb-exclusive")*/
//#pragma comment(linker, "/STACK:367077216")
#define _CRT_SECURE_NO_WARNINGS
#include <chrono>
#include <set>
#include <map>
#include <deque>
#include <string>
#include <cstdint>
#include <cmath>
#include <queue>
#include <cassert>
#include <random>
#include <bitset>
#include <iomanip>
#include <numeric>
#include <time.h>//////////////
#include <ctime>
#include <string>
#include <cstdio>
#include <vector>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
//#define endl '\n'
#define mp make_pair
#define pbc push_back
#define pob pop_back()
#define empb emplace_back
#define queuel queue<long long>
#define sqr(a) ((a) * (a))
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pin(p) cin >> p.first >> p.second;
#define uniq(a) sort(all(a));(a).resize(unique(all(a)) - a.begin());
#define rev(v) reverse(v.begin(), v.end());
#define sout(s, c) for (auto i : s) cout << i << c;
#define pout(p) cout << p.first << " " << p.second;
#define er(v, l, r) erase(v.begin() + l, v.begin() + r);
#define vin(v) for (ll i = 0; i < v.size(); ++i) cin >> v[i];
#define vout(v, c) for (int i = 0; i < v.size(); ++i) cout << v[i] << c;
#define pushi(v, a) for (int i = 0; i < a.size(); ++i) v.push_back(a[i]);
#define fastio() ios_base::sync_with_stdio(0); cout.tie(0); cin.tie(0); srand(time(NULL))
#define dab(v) for(auto i:v)cout<<i<<' ';
#define sp system("pause")
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
using namespace std;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
const ld EPS = 0.001;
const ld EPS2 = 1e-6;
const ld PI = acos(-1);
const int MOD7 = 1000000007;
const int MOD9 = 1000000009;
const int a228 = 18;
const int inf = 1e9;
const ll kekmod = 1791791791;
const ll bestmod = 1148822869;
const int MAXH = 10;
const int MAXW = 1000;
int pref[MAXH][MAXW];
int k;
int solve(vector<vector<int>> a)
{
	vector<int> sum(a.size());
	int ans = 0;
	int len = 0;
	for (int i = 0; i < a[0].size(); ++i)
	{
		for (int j = 0; j < a.size(); ++j)
		{
			sum[j] += a[j][i];
		}
		++len;
		for (int j = 0; j < a.size(); ++j)
		{
			if (sum[j] > k)
			{
				if (len == 1) return inf;
			}
		}
		for (int j = 0; j < a.size(); ++j)
		{
			if (sum[j] > k)
			{
				ans++;
				len = 0;
				for (int kk = 0; kk < a.size(); ++kk)
				{
					sum[kk] = 0;
				}
				i--;
				break;
			}
		}
	}
	return ans;
}
int get(int x1, int y1, int x2, int y2)
{
	return pref[x2][y2] - (x1 == 0 ? 0 : pref[x1 - 1][y2]) - (y1 == 0 ? 0 : pref[x2][y1 - 1]) + (x1 == 0 || y1 == 0 ? 0 : pref[x1 - 1][y1 - 1]);
}
int ppc(int n)
{
	int ans = 0;
	for (; n; n &= (n - 1)) ans++;
	return ans;
}
signed main()
{
	fastio();
	int h, w;
	cin >> h >> w >> k;
	vector<vector<char>> a(h, vector<char>(w));
	for (int i = 0; i < h; ++i)
	{
		for (int j = 0; j < w; ++j)
		{
			cin >> a[i][j];
		}
	}
	for (int i = 0; i < h; ++i)
	{
		for (int j = 0; j < w; ++j)
		{
			pref[i][j] = (i == 0 ? 0 : pref[i - 1][j]) + (j == 0 ? 0 : pref[i][j - 1]) - (i == 0 || j == 0 ? 0 : pref[i - 1][j - 1]) + a[i][j] - '0';
		}
	}
	int ans = inf;
	for (int mask = 0; mask < (1 << (h - 1)); ++mask)
	{
		vector<vector<int>> as = { {} };
		for (int bt = 0; bt <  h-1; ++bt)
		{
			if (mask & (1 << bt)) as.pbc({});
		}
		for (int j = 0; j < w; ++j)
		{
			int cnt = 0;
			int lb = -1;
			for (int bt = 0; bt < h - 1; ++bt)
			{
				if (mask & (1 << bt))
				{
					as[cnt++].pbc(get(lb + 1, j, bt, j));
					lb = bt;
				}
			}
			as[cnt++].pbc(get(lb + 1, j, h - 1, j));
		}
		ans = min(ans, solve(as) + ppc(mask));
	}
	cout << ans;
	//sp; 
}

Measured memory_bytes: 384