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 Scala (2.11.7) (judge verdict: Accepted)

object Main {

  // 隣り合うタイルの色を異なる色にする
  // 以下の４通りにする タイルの枚数は10^5,
  // S^2回で10^10なので不可,
  // SlogSなら10^6なので間に合う
  // 0101...01|0
  // 1010...10|1
  // 一回舐めて、最初が黒のパターンと最初が白のパターンで何回変更するかを調べる
  // O(2n)

  val Black = '0'
  val White = '1'

  def main(args: Array[String]): Unit = {
    import scala.io.StdIn._
    val S: List[Char] = readLine().toList

    // 最小を調べる
    def solve(): Int = {
      def change(color: Char): Char =
        if (color == Black) White else Black

      // 前が0なら今回は1にして,前が1なら今回は0にする
      @scala.annotation.tailrec
      def count(lst: List[Char], before: Char, acm: Int): Int =
        if (lst.isEmpty) acm
        else if (before == lst.head) count(lst.tail, change(lst.head), acm + 1)
        else count(lst.tail, lst.head, acm)

      Math.min(count(S.tail, S.head, 0), count(S.tail, change(S.head), 1))
    }

    println(solve())
  }

}