Problem:
Score : 500 points

Problem Statement

We have a chocolate bar partitioned into H horizontal rows and W vertical columns of squares.

The square (i, j) at the i-th row from the top and the j-th column from the left is dark if S_{i,j} is 0, and white if S_{i,j} is 1.

We will cut the bar some number of times to divide it into some number of blocks. In each cut, we cut the whole bar by a line running along some boundaries of squares from end to end of the bar.

How many times do we need to cut the bar so that every block after the cuts has K or less white squares?

Constraints

1 \leq H \leq 10

1 \leq W \leq 1000

1 \leq K \leq H \times W

S_{i,j} is 0 or 1.

Input

Input is given from Standard Input in the following format:

H W K
S_{1,1}S_{1,2}...S_{1,W}
:
S_{H,1}S_{H,2}...S_{H,W}

Output

Print the number of minimum times the bar needs to be cut so that every block after the cuts has K or less white squares.

Sample Input 1

3 5 4
11100
10001
00111

Sample Output 1

2

For example, cutting between the 1-st and 2-nd rows and between the 3-rd and 4-th columns - as shown in the figure to the left - works.

Note that we cannot cut the bar in the ways shown in the two figures to the right.

Sample Input 2

3 5 8
11100
10001
00111

Sample Output 2

0

No cut is needed.

Sample Input 3

4 10 4
1110010010
1000101110
0011101001
1101000111

Sample Output 3

3

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::vec::Vec;
use std::cmp::*;

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>().split_whitespace().map(|e| e.parse().ok().unwrap()).collect()
}

fn read_touple<T: std::str::FromStr + Copy>() -> (T, T, T) {
  let v: Vec<T> = read_vec();
  assert_eq!(v.len(), 3);
  (v[0], v[1], v[2])
}

fn main() {
    let (h, w, k) = read_touple::<usize>();
    let mut grid = vec![];
    for _ in 0..h {
      let t: Vec<char> = read::<String>().chars().collect();
      grid.push(t);
    }
    let mut ans: usize = h*w+1;

    let set = BitSet::empty(h-1);
    for b in set {
      let mut sum = vec![0; h+1];
      let bv = b.to_vec();
      let mut tot = bv.iter().fold(0, |acc, &x| if x == 1 { acc + x } else { acc });
      for i in 0..w {
        let t = calc(&grid, &bv, h, i);
        let mut ok = true;
        for i in 0..t.len() {
          if t[i] > k {
            tot = h*w+1;
            break;
          }
        }
        for i in 0..t.len() {
          sum[i] += t[i];
        }

        for i in 0..t.len() {
          if sum[i] > k {
             reset(&mut sum, &t);
             tot += 1;
             break;
          }
        }
  
      }
      if tot < ans {
        ans = tot;
        //println!("{}", tot);
        //println!("{:?}", bv);
      }
    } 

    println!("{}", ans);
}

fn reset(v :&mut Vec<usize>, t: &Vec<usize>) {
  for i in 0..t.len() {
    v[i] = t[i];
  }
}

fn calc(grid: &Vec<Vec<char>>, bv: &Vec<usize>, h: usize, c: usize) -> Vec<usize> {
    let mut res = vec![];
    let mut now = 0;

    if grid[0][c] == '1' {
       now += 1;
    }

    for i in 1..h {
      if bv[i-1] == 1 {
        res.push(now);
         now = 0;       
      }
      if grid[i][c] == '1' {
         now += 1;
      }
    }
    res.push(now);
    //println!("{:?}", res);
    res
}


#[derive(Clone, Copy, Eq, PartialEq, Ord, PartialOrd)]
struct BitSet {
    bits: usize,
    size: usize,
}

impl std::fmt::Debug for BitSet {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "BitSet {{ bits: {:b}, size: {} }}", self.bits, self.size)
    }
}

impl BitSet {
    //
    fn empty(size: usize) -> BitSet {
        if size > 64 {
            panic!("large set size")
        }
        BitSet {
            bits: 0,
            size: size,
        }
    }
    // コンンストラクター
    fn new(n: usize, size: usize) -> BitSet {
        BitSet {
            bits: n,
            size: size,
        }
    }

    // 和集合を求める
    fn union(&self, other: BitSet) -> BitSet {
        BitSet {
            bits: self.bits | other.bits,
            size: self.size, // debug
        }
    }

    // 共通部分を求める
    fn intersect(&self, other: BitSet) -> BitSet {
        BitSet {
            bits: self.bits & other.bits,
            size: self.size, // debug
        }
    }

    // n を含むかどうか
    fn include(&self, n: usize) -> bool {
        if self.size < n {
            return false;
        }
        self.bits & (1 << n) != 0
    }

    fn to_vec(&self) -> std::vec::Vec<usize> {
        let mut res = std::vec::Vec::new();
        let mut bits = self.bits;

        for _ in 0..self.size {
            res.push(bits % 2);
            bits /= 2;
        }
        res
    }

    // 集合を表すusizeの値を返す
    fn unwrap(&self) -> usize {
        self.bits
    }
}

impl Iterator for BitSet {
    type Item = BitSet;

    fn next(&mut self) -> Option<Self::Item> {
        if (1 << self.size) <= self.bits {
            return None;
        }
        self.bits += 1;
        Some(BitSet {
            bits: self.bits - 1,
            size: self.size,
        })
    }
}