#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <limits>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <functional>
#include <iomanip>

using namespace std;

#ifdef LOCAL
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#else
#define eprintf(...) 42
#endif
using ll = long long;
using vll = std::vector<ll>;
using vvll = std::vector<vll>;
using vvvll = std::vector<vvll>;
#define reps(i, S, E) for (ll i = (S); i <= (E); i++)
#define rep(i, N) reps(i, 0, N-1)
#define deps(i, E, S) for (ll i = (E); i >= (S); i--)
#define dep(i, N) deps(i, N-1, 0)
const ll INF = 1LL << 60;
const int INF_INT = 1 << 30;

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; }
struct mll {
	static ll MOD;
	ll val;
	mll(ll v = 0) : val(v % MOD) { if (val < 0) val += MOD; }
	mll operator - () const { return -val; }
	mll operator + (const mll &b) const { return val + b.val; }
	mll operator - (const mll &b) const { return val - b.val; }
	mll operator * (const mll &b) const { return val * b.val; }
	mll operator / (const mll &b) const { return mll(*this) /= b; }
	mll operator + (ll b) const { return *this + mll(b); }
	mll operator - (ll b) const { return *this - mll(b); }
	mll operator * (ll b) const { return *this * mll(b); }
	friend mll operator + (ll a, const mll &b) { return b + a; }
	friend mll operator - (ll a, const mll &b) { return -b + a; }
	friend mll operator * (ll a, const mll &b) { return b * a; }
	mll &operator += (const mll &b) { val = (val + b.val) % MOD; return *this; }
	mll &operator -= (const mll &b) { val = (val + MOD - b.val) % MOD; return *this; }
	mll &operator *= (const mll &b) { val = (val*b.val) % MOD; return *this; }
	mll &operator /= (const mll &b) {
		ll c = b.val, d = MOD, u = 1, v = 0;
		while (d) {
			ll t = c / d;
			c -= t * d; swap(c, d);
			u -= t * v; swap(u, v);
		}
		val = val * u % MOD;
		if (val < 0) val += MOD;
		return *this;
	}
	mll &operator += (ll b) { return *this += mll(b); }
	mll &operator -= (ll b) { return *this -= mll(b); }
	mll &operator *= (ll b) { return *this *= mll(b); }
	mll &operator /= (ll b) { return *this /= mll(b); }
	bool operator == (const mll &b) { return val == b.val; }
	bool operator != (const mll &b) { return val != b.val; }
	bool operator == (ll b) { return *this == mll(b); }
	bool operator != (ll b) { return *this != mll(b); }
	friend bool operator == (ll a, const mll &b) { return mll(a) == b.val; }
	friend bool operator != (ll a, const mll &b) { return mll(a) != b.val; }
	friend ostream &operator << (ostream &os, const mll &a) { return os << a.val; }
	friend istream &operator >> (istream &is, mll &a) { return is >> a.val; }
	static mll Combination(ll a, ll b) {
		chmin(b, a - b);
		if (b < 0) return mll(0);
		mll c = 1;
		rep(i, b) c *= a - i;
		rep(i, b) c /= i + 1;
		return c;
	}
};
using vmll = std::vector<mll>;
using vvmll = std::vector<vmll>;
using vvvmll = std::vector<vvmll>;
using vvvvmll = std::vector<vvvmll>;

struct Fast {
	Fast() {
		cin.tie(0);
		ios::sync_with_stdio(false);
		cout << fixed << setprecision(std::numeric_limits<double>::max_digits10);
	}
} fast; //cin,cout高速化のおまじない＋桁数指定

ll mll::MOD = (ll)(1e9 + 7);// 998244353ll;

ll LLpow(ll x, ll n) {
	ll ret = 1;
	while (n > 0) {
		if (n & 1) {
			ret *= x;
		}
		x *= x;
		n >>= 1;
	}
	return ret;
}

int main() {
	ll H;
	cin >> H;
	ll W;
	cin >> W;
	ll K;
	cin >> K;
	vector<string> S(H);
	rep(i, H) {
		cin >> S[i];
	}

	vvll T(H, vll(W, 0));
	rep(i, H) rep(j, W) {
		T[i][j] = S[i][j] - '0';
	}

	rep(i, H) {
		reps(j, 1, W - 1) {
			T[i][j] += T[i][j - 1];
		}
	}

	ll cut = H - 1;
	ll cutPat = 1ll << cut;
	vector<bool> bit(cut);
	ll result = INF;
	vll index(H, 0);

	rep(i, cutPat) {
		ll count = 0;
		ll p = 1;

		rep(j, cut) {
			if ((i & (1ll << j)) != 0ll) {
				bit[j] = true;
				count++;
				p++;
			}
			else {
				bit[j] = false;
			}
		}

		//proccess
		index[0] = 0;
		reps(j, 1, H - 1) {
			if (bit[j - 1]) {
				//cut
				index[j] = index[j - 1] + 1;
			}
			else {
				index[j] = index[j - 1];
			}
		}

		//eprintf("\nindex:");
		//rep(k, H) {
		//	eprintf("%lld ", index[k]);
		//}
		//eprintf("\n");

		bool ng = false;
		vll white(p, 0);

		rep(j, W) {
			vll curr(p, 0);

			rep(k, H) {
				if (S[k][j] == '1') {
					curr[index[k]]++;

					if (curr[index[k]] > K) {
						//1列だけで超えてしまうのでNG
						ng = true;
					}
				}
			}

			//eprintf("\ncurr:");
			//rep(k, p) {
			//	eprintf("%lld ", curr[k]);
			//}
			//eprintf("\n");

			if (ng) {
				break;
			}

			//各ピースの白個数を加えられるか判定
			bool on = false;
			rep(k, p) {
				if (white[k] + curr[k] > K) {
					//超えるので要カット
					//eprintf("\ncut!\n");
					on = true;
					count++;

					//この列だけの白個数でリセット
					rep(u, p) {
						white[u] = curr[u];
					}
					
					break;
				}
			}

			if (!on) {
				//超えないのでカットなし
				rep(u, p) {
					white[u] += curr[u];
				}
			}

			//eprintf("\nwhite:");
			//rep(k, p) {
			//	eprintf("%lld ", white[k]);
			//}
			//eprintf("\n");
		}

		if (!ng) {
			chmin(result, count);
		}
	}

	cout << result << endl;

	return 0;
}


Measured memory_bytes: 384