Problem:
Score : 300 points

Problem Statement

N tiles are arranged in a row from left to right. The initial color of each tile is represented by a string S of length N.

The i-th tile from the left is painted black if the i-th character of S is 0, and painted white if that character is 1.

You want to repaint some of the tiles black or white, so that any two adjacent tiles have different colors.

At least how many tiles need to be repainted to satisfy the condition?

Constraints

1 \leq |S| \leq 10^5

S_i is 0 or 1.

Input

Input is given from Standard Input in the following format:

S

Output

Print the minimum number of tiles that need to be repainted to satisfy the condition.

Sample Input 1

000

Sample Output 1

1

The condition can be satisfied by repainting the middle tile white.

Sample Input 2

10010010

Sample Output 2

3

Sample Input 3

0

Sample Output 3

0

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

#![allow(unused)]

use std::{cell, cmp, collections, fmt, iter, ops};

use cell::{Cell, RefCell};
use cmp::Ordering;
use collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, LinkedList, VecDeque};
use fmt::{Debug, Display};
use ops::{
    Add, AddAssign, Deref, DerefMut, Div, DivAssign, Mul, MulAssign, Neg, Not, Sub, SubAssign,
};
use std::str::FromStr;



// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[macro_export]
macro_rules! input {
    (src = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        _input_inner!(iter, $($r)*)
    };
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        input!(src = s, $($r)*)
    };
}

#[macro_export]
macro_rules! inputln {
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_line(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        _input_inner!(iter, $($r)*)
    };
}

#[macro_export]
macro_rules! _input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = _parse_value!($iter, $t);
        _input_inner!($iter $($r)*)
    };

    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = _parse_value!($iter, $t);
        _input_inner!($iter $($r)*)
    };
}

#[macro_export]
macro_rules! _parse_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(_parse_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| _parse_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, chars) => {
        _parse_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        _parse_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().expect("No item left").parse::<$t>().expect("Parse error")
    };
}

#[cfg(feature = "debug")]
#[macro_export]
macro_rules! d {
    ($val:expr) => {
        match $val {
            tmp => {
                eprintln!("[{}] {} = {:#?}", line!(), stringify!($val), &tmp);
                tmp
            }
        }
    };
}

#[cfg(not(feature = "debug"))]
#[macro_export]
macro_rules! d {
    ($val:expr) => {
        $val
    };
}

/// ```
/// # use lib::*;
/// assert_eq!(vmin!(0, 1), 0);
/// assert_eq!(vmin!(3, 2, 1), 1);
/// ```
#[macro_export]
macro_rules! vmin {
    ($x:expr, $y:expr) => { std::cmp::min($x, $y) };
    ($x:expr, $($args:expr),*) => { std::cmp::min($x, vmin!($($args),*)) };
}

/// ```
/// # use lib::*;
/// assert_eq!(vmax!(0, 1), 1);
/// assert_eq!(vmax!(3, 2, 1), 3);
/// ```
#[macro_export]
macro_rules! vmax {
    ($x:expr, $y:expr) => { std::cmp::max($x, $y) };
    ($x:expr, $($args:expr),*) => { std::cmp::max($x, vmax!($($args),*)) };
}

/// ```
/// # use lib::*;
/// let v = vec![2, 1, 0, 3];
/// assert_eq!(prefix!(v).collect::<Vec<i32>>(), vec![0, 2, 3, 3, 6]);
/// ```
#[macro_export]
macro_rules! prefix {
    ($iter: expr) => {{
        use std::iter;

        iter::once(Default::default())
            .chain($iter.into_iter())
            .scan(Default::default(), |acc, x| {
                *acc += x;
                Some(*acc)
            })
    }};
}

pub mod num {
    /// ```
    /// # use lib::*;
    /// assert_eq!(num::gcd(2, 5), 1);
    /// assert_eq!(num::gcd(4, 6), 2);
    /// ```
    pub fn gcd(a: u64, b: u64) -> u64 {
        if b == 0 {
            a
        } else {
            gcd(b, a % b)
        }
    }

    /// ```
    /// # use lib::*;
    /// assert_eq!(num::gcd_slice(&[3]), 3);
    /// assert_eq!(num::gcd_slice(&[2, 5]), 1);
    /// assert_eq!(num::gcd_slice(&[4, 6, 8]), 2);
    /// ```
    pub fn gcd_slice(a: &[u64]) -> u64 {
        a.iter().fold(a[0], |a, b| gcd(a, *b))
    }

    /// ```
    /// # use lib::*;
    /// assert_eq!(num::lcm(2, 5), 10);
    /// assert_eq!(num::lcm(4, 6), 12);
    /// ```
    pub fn lcm(a: u64, b: u64) -> u64 {
        a * b / gcd(a, b)
    }

    /// ```
    /// # use lib::*;
    /// assert_eq!(num::lcm_slice(&[3]), 3);
    /// assert_eq!(num::lcm_slice(&[2, 5]), 10);
    /// assert_eq!(num::lcm_slice(&[4, 6, 8]), 24);
    /// ```
    pub fn lcm_slice(a: &[u64]) -> u64 {
        a.iter().fold(a[0], |a, b| lcm(a, *b))
    }

    /// ```
    /// # use lib::num::*;
    /// assert_eq!(next_prime(0), 2);
    /// assert_eq!(next_prime(1), 2);
    /// assert_eq!(next_prime(2), 3);
    /// assert_eq!(next_prime(3), 5);
    /// ```
    pub fn next_prime(mut x: u64) -> u64 {
        while {
            x += 1;
            !is_prime(x)
        } {}
        x
    }

    /// ```
    /// # use lib::num::*;
    /// assert!(!is_prime(0));
    /// assert!(!is_prime(1));
    /// assert!(is_prime(2));
    /// assert!(is_prime(3));
    /// assert!(!is_prime(4));
    /// ```
    pub fn is_prime(x: u64) -> bool {
        match x {
            0 | 1 => false,
            2 => true,
            _ => {
                let ceil = (x as f64).sqrt().ceil() as u64;
                (2..(ceil + 1)).all(|i| x % i != 0)
            }
        }
    }

    /// ```
    /// # use lib::num::*;
    /// assert_eq!(
    ///     factorize(2 * 2 * 3 * 4 * 5 * 5 * 7 * 9),
    ///     vec![0, 0, 4, 3, 0, 2, 0, 1]
    /// );
    /// ```
    pub fn factorize(mut x: u64) -> Vec<usize> {
        let mut pows = vec![];
        let mut prime = 1_u64;
        while {
            prime = next_prime(prime);
            prime <= x
        } {
            while x % prime == 0 {
                while pows.len() <= prime as usize {
                    pows.push(0);
                }
                pows[prime as usize] += 1;
                x /= prime;
            }
        }

        pows
    }
}

pub mod bits {
    /// ```
    /// # use lib::*;
    /// assert!(!bits::is_high(0b0100, 0));
    /// assert!(bits::is_high(0b0100, 2));
    /// ```
    pub fn is_high(bits: u64, pos: usize) -> bool {
        bits & (1 << pos as u64) != 0
    }

    /// ```
    /// # use lib::*;
    /// assert_eq!(bits::len(0b1010), 4);
    /// assert_eq!(bits::len(0b1), 1);
    /// assert_eq!(bits::len(0b0), 0);
    /// ```
    pub fn len(bits: u64) -> usize {
        for b in 0..64 {
            if is_high(bits, 63 - b) {
                return (64 - b) as usize;
            }
        }

        return 0;
    }
}

#[derive(Clone, Debug)]
pub struct UnionFind {
    parent: Vec<usize>,
    size: Vec<usize>,
    depth: Vec<usize>,
}

/// ```
/// # use lib::*;
/// let mut uf = UnionFind::new(4);
/// assert_eq!(uf.size(0), 1);
/// assert_eq!(uf.find(0), 0);
///
/// uf.union(0, 1);
/// assert!(uf.same(0, 1));
/// assert!(uf.find(0) == 0 || uf.find(0) == 1);
/// assert_eq!(uf.size(0), 2);
///
/// uf.union(1, 2);
/// assert!(uf.same(0, 2));
/// assert!(uf.find(0) == 0 || uf.find(0) == 1); // not 2
/// assert_eq!(uf.size(0), 3);
/// ```
impl UnionFind {
    pub fn new(capacity: usize) -> Self {
        UnionFind {
            parent: (0..capacity).collect(),
            size: vec![1; capacity],
            depth: vec![0; capacity],
        }
    }

    pub fn size(&mut self, a: usize) -> usize {
        let p = self.find(a);
        self.size[p]
    }

    pub fn find(&mut self, a: usize) -> usize {
        if self.parent[a] == a {
            a
        } else {
            let p = self.parent[a];
            self.parent[a] = self.find(p);
            self.parent[a]
        }
    }

    pub fn same(&mut self, a: usize, b: usize) -> bool {
        self.find(a) == self.find(b)
    }

    pub fn union(&mut self, a: usize, b: usize) {
        let a = self.find(a);
        let b = self.find(b);
        if a == b {
            return;
        }

        if self.depth[a] < self.depth[b] {
            self.parent[a] = b;
            self.size[b] += self.size[a];
        } else {
            self.parent[b] = a;
            self.size[a] += self.size[b];

            if self.depth[a] == self.depth[b] {
                self.depth[a] += 1;
            }
        }
    }
}

// std::cmp::Reverse is stable since 1.19
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub struct Rev<T>(T);

impl<T: PartialOrd> PartialOrd for Rev<T> {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T: Ord> Ord for Rev<T> {
    fn cmp(&self, other: &Self) -> Ordering {
        other.0.cmp(&self.0)
    }
}

/// ```
/// # use lib::*;
/// use std::collections::BinaryHeap;
///
/// let heap = [1, 0, 2, 3].iter().cloned().collect::<BinaryHeap<_>>();
/// let sorted = heap.into_iter_sorted().collect::<Vec<_>>();;
/// assert_eq!(sorted, vec![3, 2, 1, 0]);
/// ```
// BinaryHeap::iter() iterates values in arbitrary order.
// std::iter::from_fn() is stable since 1.34
pub struct IterSorted<T: Ord> {
    heap: BinaryHeap<T>,
}

pub trait IntoIteratorSorted {
    type Item;
    type IntoIterSorted: Iterator<Item = Self::Item>;

    fn into_iter_sorted(self) -> Self::IntoIterSorted;
}

impl<T: Ord> IntoIteratorSorted for BinaryHeap<T> {
    type Item = T;
    type IntoIterSorted = IterSorted<T>;

    fn into_iter_sorted(self) -> IterSorted<T> {
        IterSorted { heap: self }
    }
}

impl<T: Ord> Iterator for IterSorted<T> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        self.heap.pop()
    }
}

pub trait Digits {
    fn digits(self, radix: Self) -> usize;
}

macro_rules! impl_digits_int {
    ($int:ty) => {
        impl Digits for $int {
            fn digits(mut self, radix: Self) -> usize {
                let mut ans = 0;
                while self > 0 {
                    ans += 1;
                    self /= radix;
                }
                ans
            }
        }
    };
}

impl_digits_int!(i8);
impl_digits_int!(u8);
impl_digits_int!(i16);
impl_digits_int!(u16);
impl_digits_int!(i32);
impl_digits_int!(u32);
impl_digits_int!(i64);
impl_digits_int!(u64);
impl_digits_int!(isize);
impl_digits_int!(usize);

fn main() {
    input!(s: chars);
    let tiles = s
        .into_iter()
        .map(|c| c.to_digit(10).expect("failed to parse digit"))
        .collect::<Vec<_>>();

    let ans = cmp::min(
        tiles
            .iter()
            .zip([0, 1].iter().cycle())
            .map(|(t, z)| t == z)
            .filter(|b| *b)
            .count(),
        tiles
            .iter()
            .zip([1, 0].iter().cycle())
            .map(|(t, z)| t == z)
            .filter(|b| *b)
            .count(),
    );

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

#[cfg(test)]
mod tests {
    #[allow(unused)]
    use super::*;

    #[test]
    fn test() {
        //
    }
}