#include "bits/stdc++.h"
using namespace std;
const int MAX = 700000;
const int MOD = 1000000007;

long long  fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
	fac[0] = fac[1] = 1;
	finv[0] = finv[1] = 1;
	inv[1] = 1;
	for (int i = 2; i < MAX; i++) {
		fac[i] = fac[i - 1] * i % MOD;
		inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
		finv[i] = finv[i - 1] * inv[i] % MOD;
	}
}

// 二項係数計算
long long COM(int n, int k) {
	if (n < k) return 0;
	if (n < 0 || k < 0) return 0;
	return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
/*第二引数で第一引数を割ったときの切り上げの計算*/
long long int maxtime(long long int x, long long int y){
	return(x + y - 1) / y;

}
/*最大公約数*/
long long int lcm(long long int number1, long long int number2) {
	long long int m = number1;
	long long int n = number2;

	if (number2 > number1) {
		m = number2;
		n = number1;
	}
	long long int s = -1;
	while (s != 0) {
		s = m % n;
		m = n;
		n = s;
	}
	return m;
}
/*最大公倍数*/
long long int gcd(long long int number1, long long int number2) {
	long long int m = number1;
	long long int n = number2;
	return m / lcm(m, n) * n;
}
/*逆元計算*/
long long int  modinv(long long a, long long m) {
	long long int b = m, u = 1, v = 0;
	while (b) {
		long long int t = a / b;
		a -= t * b; swap(a, b);
		u -= t * v; swap(u, v);
	}
	u %= m;
	if (u < 0) u += m;
	return u;
}
// index が条件を満たすかどうか
vector<long long int >meguru;
bool isOK(int index, int key) {
	if (meguru[index] >= key) return true;
	else return false;
}
// 汎用的な二分探索のテンプレ
int binary_search(int key) {
	int left = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1
	int right = (int)meguru.size(); // 「index = a.size()-1」が条件を満たさないこともあるので、初期値は a.size()
	/* どんな二分探索でもここの書き方を変えずにできる！ */
	while (right - left > 1) {
		int mid = left + (right - left) / 2;

		if (isOK(mid, key)) right = mid;
		else left = mid;
	}
	/* left は条件を満たさない最大の値、right は条件を満たす最小の値になっている */
	return right;
}
long long modpow(long long a, long long n, long long mod) {
	long long res = 1;
	while (n > 0) {
		if (n & 1) res = res * a % mod;
		a = a * a % mod;
		n >>= 1;
	}
	return res;
}
int main() {
	int h, w,k;
	cin >> h >> w >> k;
	int dp[10] = {};
	vector<vector<int>>s(20);
	for (int i = 0; i < h; i++) {
		s[i].push_back(0);
	}
	for (int i = 0; i < h; i = i + 1) {
		for (int j = 0; j < w; j = j + 1) {
			char a;
			cin >> a;
			if (a == '1') {
				s[i].push_back(s[i][j]+1);
			}
			else {
				s[i].push_back(s[i][j]);
			}
		}
	}

	int ans = 100000000;
	for (int i = 0; i < pow(2, h - 1); i++) {
		int kat = 0;
		for (int j = 0; j < h - 1; j++) {
			if (dp[j] == 2) {
				dp[j + 1] = dp[j + 1] + 1;
				dp[j] = 0;
			}
		}
		for (int j = 0; j < h - 1; j++) {
			if (dp[j] == 1) {
				kat++;
			}
		}
		int sum = 0;
		int a = 0;
		int c = 0;
		for (int m = 0; m < w; m++) {
			for (int l = 0; l < h; l++) {
				sum = sum + s[l][m+1] - s[l][a];
				if (sum > k) {
					if (m + 1 - a == 1) {
						c = 1;
					}
					a = m;
					m = m - 1;
					sum = 0;
					kat++;
					break;
				}
				if (dp[l] == 1) {
					sum = 0;
				}
				if (l == h - 1) {
					sum = 0;
				}
			}
			if (c == 1) {
				break;
			}
			if (m == w - 1) {
				ans = min(ans, kat);
			}
		}
		dp[0] = dp[0] + 1;
	}
	cout << ans;
}


Measured memory_bytes: 256