#![allow(unused_imports)]
#![allow(non_snake_case, unused)]

use std::cmp::*;
use std::collections::*;
use std::ops::*;

macro_rules! eprint {
	($($t:tt)*) => {{
		use ::std::io::Write;
		let _ = write!(::std::io::stderr(), $($t)*);
	}};
}
macro_rules! eprintln {
	() => { eprintln!(""); };
	($($t:tt)*) => {{
		use ::std::io::Write;
		let _ = writeln!(::std::io::stderr(), $($t)*);
	}};
}
macro_rules! dbg {
	($v:expr) => {{
		let val = $v;
		eprintln!("[{}:{}] {} = {:?}", file!(), line!(), stringify!($v), val);
		val
	}}
}

macro_rules! mat {
	($($e:expr),*) => { Vec::from(vec![$($e),*]) };
	($($e:expr,)*) => { Vec::from(vec![$($e),*]) };
	($e:expr; $d:expr) => { Vec::from(vec![$e; $d]) };
	($e:expr; $d:expr $(; $ds:expr)+) => { Vec::from(vec![mat![$e $(; $ds)*]; $d]) };
}

macro_rules! ok {
	($a:ident$([$i:expr])*.$f:ident()$(@$t:ident)*) => {
		$a$([$i])*.$f($($t),*)
	};
	($a:ident$([$i:expr])*.$f:ident($e:expr$(,$es:expr)*)$(@$t:ident)*) => { {
		let t = $e;
		ok!($a$([$i])*.$f($($es),*)$(@$t)*@t)
	} };
}

pub fn readln() -> String {
	let mut line = String::new();
	::std::io::stdin().read_line(&mut line).unwrap_or_else(|e| panic!("{}", e));
	line
}

macro_rules! read {
	($($t:tt),*; $n:expr) => {{
		let stdin = ::std::io::stdin();
		let ret = ::std::io::BufRead::lines(stdin.lock()).take($n).map(|line| {
			let line = line.unwrap();
			let mut it = line.split_whitespace();
			_read!(it; $($t),*)
		}).collect::<Vec<_>>();
		ret
	}};
	($($t:tt),*) => {{
		let line = readln();
		let mut it = line.split_whitespace();
		_read!(it; $($t),*)
	}};
}

macro_rules! _read {
	($it:ident; [char]) => {
		_read!($it; String).chars().collect::<Vec<_>>()
	};
	($it:ident; [u8]) => {
		Vec::from(_read!($it; String).into_bytes())
	};
	($it:ident; usize1) => {
		$it.next().unwrap_or_else(|| panic!("input mismatch")).parse::<usize>().unwrap_or_else(|e| panic!("{}", e)) - 1
	};
	($it:ident; [usize1]) => {
		$it.map(|s| s.parse::<usize>().unwrap_or_else(|e| panic!("{}", e)) - 1).collect::<Vec<_>>()
	};
	($it:ident; [$t:ty]) => {
		$it.map(|s| s.parse::<$t>().unwrap_or_else(|e| panic!("{}", e))).collect::<Vec<_>>()
	};
	($it:ident; $t:ty) => {
		$it.next().unwrap_or_else(|| panic!("input mismatch")).parse::<$t>().unwrap_or_else(|e| panic!("{}", e))
	};
	($it:ident; $($t:tt),+) => {
		($(_read!($it; $t)),*)
	};
}

pub fn main() {
	let _ = ::std::thread::Builder::new().name("run".to_string()).stack_size(32 * 1024 * 1024).spawn(run).unwrap().join();
}

const MOD: usize = 1_000_000_007;
const INF: i64 = std::i64::MAX/2;

pub trait SetMinMax {
	fn setmin(&mut self, v: Self) -> bool;
	fn setmax(&mut self, v: Self) -> bool;
}
impl<T> SetMinMax for T where T: PartialOrd {
	fn setmin(&mut self, v: T) -> bool {
		*self > v && { *self = v; true }
	}
	fn setmax(&mut self, v: T) -> bool {
		*self < v && { *self = v; true }
	}
}

fn solve() {
	let (h,w,K) = read!(usize,usize,i64);
	let b = read!(String;h);
	let mut board = vec![vec![0 as i64;w];h];
	for i in 0..h {
		for (j,c) in b[i].chars().enumerate() {
			if c=='1'{
				board[i][j] = 1;
			}
		}
	}
	let mut cum = vec![vec![0 as i64;w+1];h+1];
	for i in 0..h {
		for j in 0..w {
			cum[i+1][j+1] = board[i][j] + cum[i+1][j] + cum[i][j+1] - cum[i][j];
		}
	}
	let mut ans = h+w;
	for i in 0..(1<<h) {
		let mut v = vec![];
		let mut flag = true;
		let mut sum = 0;
		for j in 0..h {
			if ((i>>j)%2)==1 {
				v.push(j+1);
				sum += 1;
			}
			else if j==h-1 {
				flag = false;
			}
		}
		if !flag {
			continue;
		}
		let mut pj = 0;
		let mut j = 1;
		let mut b = true;
		loop {
			let mut pk = 0;
			let mut f = true;
			for &k in &v {
				let val = cum[k][j] - cum[pk][j] - cum[k][pj] + cum[pk][pj];
				if val>K {
					f = false;
					break;
				}
				pk = k;
			}
			if !f {
				if pj==j-1 {
					b = false;
					break;
				}
				pj = j-1;
				sum += 1;
				continue;
			}
			j += 1;
			if j>w {
				break;
			}
		}
		if !b {
			continue;
		}
		ans = min(ans, sum-1);
	}
	// for i in 0..h {
	// 	for j in 0..w {
	// 		print!("{} ", cum[i+1][j+1]);
	// 	}
	// 	println!();
	// }
	println!("{}", ans);
}

fn run() {
    solve();
}


Measured memory_bytes: 8572