package main

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

var DEBUG = true

func main() {
	defer Flush()
	H := readi()
	W := readi()
	K := readi()
	S := make([][]int, H)
	for i := range S {
		b := readb()
		S[i] = make([]int, W)
		for j := range b {
			S[i][j] = int(b[j] - '0')
		}
	}

	sum := cumulativeSum2D(S)
	ans := H * W * 2

	DoWithExhaustive01(H-1, func(b []int) {
		//dbg("===============")
		//dbg("b", b)
		var tans int
		X := make([][][]int, 0)
		hs := make([]int, 0)
		hs = append(hs, 0)
		prev := 0
		for i := range b {
			if b[i] == 0 {
				continue
			}
			X = append(X, S[prev:i+1])
			prev = i + 1
			hs = append(hs, prev)
			tans++
		}
		X = append(X, S[prev:])
		hs = append(hs, H)

		//for i := range X {
		//	dbg("---")
		//	for j := range X[i] {
		//		dbg("X", X[i][j])
		//	}
		//}
		//dbg("hs", hs)
		//dbg(K, ans)

		prev = 0
		for j := 1; j <= W; j++ {
			ok := true
			for i := 0; i+1 < len(hs); i++ {
				vs := sum(hs[i], prev, hs[i+1], j)
				if K < vs {
					ok = false
					break
				}
			}
			if !ok {
				nprev := j - 1
				if nprev == prev {
					// not valid
					return
				}
				prev = nprev
				tans++
				j--
			}
		}
		ans = min(ans, tans)
	})
	println(ans)

}

// cumulativeSum returns a function func(i, j int) int64 which returns
// the sum of a[i:j]
func cumulativeSum(a []int) func(int, int) int64 {
	n := len(a)
	b := make([]int64, n+1)
	for i := 0; i < n; i++ {
		b[i+1] = int64(a[i]) + b[i]
	}
	return func(i, j int) int64 {
		return b[j] - b[i]
	}
}

// cumulativeSum2D returns a function func(i0, j0, i1, j1 int) int64
// which returns the sum of a[i0][j0:j1], a[i0+1][j0:j1], ...,
// a[i1-1][j0:j1]
func cumulativeSum2D(a [][]int) func(int, int, int, int) int {
	n := len(a)
	m := len(a[0])
	b := make([][]int, n+1)
	for i := 0; i <= n; i++ {
		b[i] = make([]int, m+1)
	}

	for i := 1; i <= n; i++ {
		for j := 1; j <= m; j++ {
			b[i][j] = int(a[i-1][j-1]) + b[i][j-1]
		}
	}
	for j := 1; j <= m; j++ {
		for i := 1; i <= n; i++ {
			b[i][j] += b[i-1][j]
		}
	}

	return func(i0, j0, i1, j1 int) int {
		return b[i1][j1] - b[i0][j1] - b[i1][j0] + b[i0][j0]
	}
}

// DoWithExhaustiveBits(n, fn) calls fn with []bool for each n-bit true/false pattern
// e.g.
// DoWithExhaustiveBits(10, func(b []bool) { fmt.Println(b) }
// -> [false, false, false, false, false, false, false, false, false, false]
//    [true, false, false, false, false, false, false, false, false, false]
//    [false, true, false, false, false, false, false, false, false, false]
//    [true, true, false, false, false, false, false, false, false, false]
//    [false, false, true, false, false, false, false, false, false, false]
//    ...
func DoWithExhaustiveBits(n int, fn func([]bool)) {
	N := 1 << uint(n)
	a := make([]bool, n)
	for i := 0; i < N; i++ {
		for j := 0; j < n; j++ {
			k := n - j - 1
			if i&(1<<uint(j)) == 0 {
				a[k] = false
			} else {
				a[k] = true
			}
		}
		fn(a)
	}
}

// DoWithExhaustive01(n, fn) calls fn with []int for each n-bit 0/1 pattern
// e.g.
// DoWithExhaustive01(10, func(b []int) { fmt.Println(b) }
// -> [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
//    [1, 0, 0, 0, 0, 0, 0, 0, 0, 0]
//    [0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
//    [1, 1, 0, 0, 0, 0, 0, 0, 0, 0]
//    [0, 0, 1, 0, 0, 0, 0, 0, 0, 0]
//    ...
func DoWithExhaustive01(n int, fn func([]int)) {
	N := 1 << uint(n)
	a := make([]int, n)
	for i := 0; i < N; i++ {
		for j := 0; j < n; j++ {
			k := n - j - 1
			if i&(1<<uint(j)) == 0 {
				a[k] = 0
			} else {
				a[k] = 1
			}
		}
		fn(a)
	}
}

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

// IO helper functions

// Returns next token from input.  It must be initialized by SetInput()
// before the first call.
var nextToken func() ([]byte, error)
var nextLine func() ([]byte, error)

// Holds a buffer for output.  It must be initialized by SetOutput().
// All IO fucntions (read*() and [e]print*()) should write to OutputWriter
// instead of this.
var OutputBuffer *bufio.Writer

// Holds an io.Writer.  It must be initialized by SetOutput()
var OutputWriter io.Writer

// Set IO functions for interactive input/output.
func SetInteractive(w io.Writer, r io.Reader) {
	SetUnbefferedInput(r)
	OutputBuffer = nil
	OutputWriter = w
}

// Setup OutputBuffer and OutputWriter.
func SetOutput(w io.Writer) {
	OutputBuffer = bufio.NewWriter(w)
	OutputWriter = OutputBuffer
}

// Flushes OutputBuffer
func Flush() {
	if OutputBuffer != nil {
		OutputBuffer.Flush()
	}
}

// Returns true if c is a white space
func IsSpace(c byte) bool {
	switch c {
	case '\t', '\n', '\v', '\f', '\r', ' ':
		return true
	}
	return false
}

func IsNewLine(c byte) bool {
	switch c {
	case '\n', '\r':
		return true
	}
	return false
}

// Setup nextToken with input buffer.
func SetInput(r io.Reader) {
	buf := new(bytes.Buffer)
	var b []byte

	var i int
	rest := func() ([]byte, error) {
		for i < len(b) && IsSpace(b[i]) {
			i++
		}
		if i == len(b) {
			return nil, io.ErrUnexpectedEOF
		}
		j := i
		for i < len(b) && !IsSpace(b[i]) {
			i++
		}
		return b[j:i], nil
	}
	initial := func() ([]byte, error) {
		io.Copy(buf, r)
		b = buf.Bytes()
		nextToken = rest
		return rest()
	}
	nextToken = initial

	restLn := func() ([]byte, error) {
		for i < len(b) && IsNewLine(b[i]) {
			i++
		}
		if i == len(b) {
			return nil, io.ErrUnexpectedEOF
		}
		j := i
		for i < len(b) && !IsNewLine(b[i]) {
			i++
		}
		return b[j:i], nil
	}

	initialLn := func() ([]byte, error) {
		io.Copy(buf, r)
		b = buf.Bytes()
		nextLine = restLn
		return restLn()
	}
	nextLine = initialLn
}

// Setup nextToken without input buffer.
func SetUnbefferedInput(r io.Reader) {
	buf := bufio.NewReader(r)
	var b []byte

	var i int
	nextToken = func() ([]byte, error) {
		var err error
		if i == len(b) {
			b, err = buf.ReadBytes('\n')
			if err != nil {
				return nil, err
			}
			i = 0
			j := len(b) - 1
			for 0 <= j && IsSpace(b[j]) {
				j--
			}
			b = b[0 : j+1]
		}
		for i < len(b) && IsSpace(b[i]) {
			i++
		}
		j := i
		for i < len(b) && !IsSpace(b[i]) {
			i++
		}
		if i == j {
			return nil, io.ErrUnexpectedEOF
		}
		return b[j:i], nil
	}
}

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

// IO functions

// Reads next token and return it as []byte
func readb() []byte {
	b, err := nextToken()
	if err != nil {
		panic(err)
	}
	return b[:len(b):len(b)]
}

// Reads next token and return it as string
func reads() string {
	return string(readb())
}

// Read next line as []byte.  Trailing '\n' will not be included.
// See also comments on readb()
func readbln() []byte {
	b, err := nextLine()
	if err != nil {
		panic(err)
	}
	return b[:len(b):len(b)]
}

// Read next line as string
func readsln() string {
	return string(readbln())
}

// Reads next token and return it as int64
func readll() int64 {
	i, err := strconv.ParseInt(reads(), 10, 64)
	if err != nil {
		panic(err.Error())
	}
	return i
}

// Reads next token and return it as int
func readi() int {
	return int(readll())
}

// Reads next token and return it as float64
func readf() float64 {
	f, err := strconv.ParseFloat(reads(), 64)
	if err != nil {
		panic(err.Error())
	}
	return f
}

// Write args to OutputWriter with the format f
func printf(f string, args ...interface{}) (int, error) {
	return fmt.Fprintf(OutputWriter, f, args...)
}

// Write args to OutputWriter without format
func println(args ...interface{}) (int, error) {
	return fmt.Fprintln(OutputWriter, args...)
}

// Write args to stderr with the format f
func eprintf(f string, args ...interface{}) (int, error) {
	return fmt.Fprintf(os.Stderr, f, args...)
}

// Write args to stderr without format
func eprintln(args ...interface{}) (int, error) {
	return fmt.Fprintln(os.Stderr, args...)
}

func dbgf(f string, args ...interface{}) {
	if !DEBUG {
		return
	}
	fmt.Fprintf(os.Stderr, f, args...)
}

func dbg(args ...interface{}) {
	if !DEBUG {
		return
	}
	fmt.Fprintln(os.Stderr, args...)
}

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

// Utilities

func sumSlice(a []int) int {
	var res int
	for _, v := range a {
		res += v
	}
	return res
}

func sumSlicell(a []int64) int64 {
	var res int64
	for _, v := range a {
		res += v
	}
	return res
}

func readInts(N int) (int, []int) {
	if N == 0 {
		N = readi()
	}
	a := make([]int, N)
	for i := range a {
		a[i] = readi()
	}
	return N, a
}

func readIntsll(N int) (int, []int64) {
	if N == 0 {
		N = readi()
	}
	a := make([]int64, N)
	for i := range a {
		a[i] = readll()
	}
	return N, a
}

// reverse slice in place
func reverse(a []int) {
	for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 {
		a[i], a[j] = a[j], a[i]
	}
}

// areverse: allocating reverse
func areverse(a []int) []int {
	b := make([]int, len(a))
	for i := range a {
		b[i] = a[len(a)-i-1]
	}
	return b
}

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

// Simple math functions

const (
	// big prime
	INF  = 1000000007
	INF2 = 1000000009
	INF3 = 998244353
)

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

func minll(a, b int64) int64 {
	if a < b {
		return a
	}
	return b
}

func minf(a, b float64) float64 {
	if a < b {
		return a
	}
	return b
}

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

func maxll(a, b int64) int64 {
	if a < b {
		return b
	}
	return a
}

func maxf(a, b float64) float64 {
	if a < b {
		return b
	}
	return a
}

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

func absll(a int64) int64 {
	if a < 0 {
		return -a
	}
	return a
}

func absf(a float64) float64 {
	if a < 0 {
		return -a
	}
	return a
}

// egcd(a, b) returns d, x, y:
//   d is gcd(a,b)
//   x, y are  integers that satisfy ax + by = d
func egcd(a, b int) (int, int, int) {
	if b == 0 {
		return a, 1, 0
	}
	d, x, y := egcd(b, a%b)
	return d, y, x - a/b*y
}

func egcdll(a, b int64) (int64, int64, int64) {
	if b == 0 {
		return a, 1, 0
	}
	d, x, y := egcdll(b, a%b)
	return d, y, x - a/b*y
}

func gcd(a, b int) int {
	d, _, _ := egcd(a, b)
	return d
}

func gcdll(a, b int64) int64 {
	d, _, _ := egcdll(a, b)
	return d
}

// set up IO functions
func init() {
	// for non-interactive
	SetInput(os.Stdin)
	SetOutput(os.Stdout)

	// Enable below when interactive.  Its ok to leave above intact.
	// SetInteractive(os.Stdout, os.Stdin)
}


Measured memory_bytes: 1024