package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"strconv"
)

const mod = 998244353
const facNum = 300001
const INF = 1 << 31

func main() {
	sc := newScanner(os.Stdin)
	H, W, K := sc.i(), sc.i(), sc.i()
	S := make([]string, H)
	for i := 0; i < H; i++ {
		S[i] = sc.s()
	}
	l := 1 << (H - 1)
	ps := make([][]int, 0)
	box := make([][]int, 0)
	ans := make([]int, 0)
	for i := 0; i < l; i++ {
		t := i
		p := make([]int, 0)
		p = append(p, 0)
		for j := 1; j < H; j++ {
			if t&1 == 1 {
				p = append(p, j)
			}
			t >>= 1
		}
		p = append(p, H)
		ps = append(ps, p)
		box = append(box, make([]int, len(p)-1))
		ans = append(ans, len(p)-2)
	}
	//ans := INF
	for i := 0; i < W; i++ {
		for pi, p := range ps {
			if ans[pi] == INF {
				continue
			}
			shikiri := false
			for j := 0; j < len(p)-1; j++ {
				white := 0
				for k := p[j]; k < p[j+1]; k++ {
					if S[k][i] == '1' {
						white++
					}
				}
				// 作れない
				if white > K {
					ans[pi] = INF
					break
				}
				if box[pi][j]+white > K {
					ans[pi]++
					shikiri = true
					break
				}
			}
			for j := 0; j < len(p)-1; j++ {
				white := 0
				for k := p[j]; k < p[j+1]; k++ {
					if S[k][i] == '1' {
						white++
					}
				}
				// 作れない
				if white > K {
					ans[pi] = INF
					break
				}
				if shikiri {
					box[pi][j] = white
				} else {
					box[pi][j] += white
				}
			}

		}
	}
	m := INF
	for _, a := range ans {
		m = min(m, a)
	}
	fmt.Println(m)
}

// fmt.Println("Yes")
// fmt.Println("No")

// I/O
type scanner struct {
	sc *bufio.Scanner
}

func newScanner(input io.Reader) *scanner {
	sc := bufio.NewScanner(input)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &scanner{sc}
}

func (s *scanner) s() string {
	s.sc.Scan()
	return s.sc.Text()
}

func (s *scanner) i() int {
	i, e := strconv.Atoi(s.s())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *scanner) f() float64 {
	f, e := strconv.ParseFloat(s.s(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *scanner) bs() []byte {
	return []byte(s.s())
}

func (s *scanner) is(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.i()
	}
	return res
}

func (s *scanner) fs(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.f()
	}
	return res
}

//------------------------------

//const factorial = new
func Pow(a, n int) int {
	ans := 1
	for n > 0 {
		if (n & 1) == 1 {
			ans = ans * a % mod
		}
		a = a * a % mod
		n = n >> 1
	}
	return ans
}

// a^{-1} (mod) を計算
func Inv(a int) int {
	return Pow(a, mod-2)
}

func Gcd(a, b int) int {
	if a < b {
		return Gcd(b, a)
	}
	for b != 0 {
		a, b = b, a%b
	}
	return a
}

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func combination(n, k int, fac, ifac []int) int {
	if n < k || n < 0 {
		return 0
	}
	if k == 0 {
		return 1
	}
	//while n != 0
	ans := ifac[k] * ifac[n-k] % mod
	return ans * fac[n] % mod
}

func abs(i int) int {
	if i < 0 {
		return -i
	}
	return i
}

// 階乗: factorial
// コンビネーションを計算する際に前もって計算しておく
// fac[k] => k! (mod M)
// ifac[k] => k!^(M-2) (mod M)
// n: ex. 2 * 10^5 => 200001
// fac, ifac := factorial()
func factorial() (fac []int, ifac []int) {
	fac = make([]int, facNum)
	fac[0] = 1
	ifac = make([]int, facNum)
	ifac[0] = 1
	for i := 1; i < facNum; i++ {
		fac[i] = fac[i-1] * i % mod
		ifac[i] = ifac[i-1] * Pow(i, mod-2) % mod
	}
	return
}

// vs: sorted slice int value
func lowerBound(vs []int, v int) (index int) {
	n := len(vs) / 2
	m := len(vs)
	for m != n {
		if vs[n] < v {
			n = (m-n+1)/2 + n
			//m = m
		} else {
			m = n
			n = n / 2
		}
	}
	return n
}

func IntSlice(n, init int) []int {
	r := make([]int, n)
	for i := range r {
		r[i] = init
	}
	return r
}


Measured memory_bytes: 1968