Problem:
Score : 300 points

Problem Statement

There are N stones arranged in a row. Every stone is painted white or black.
A string S represents the color of the stones. The i-th stone from the left is white if the i-th character of S is ., and the stone is black if the character is #.

Takahashi wants to change the colors of some stones to black or white so that there will be no white stone immediately to the right of a black stone.
Find the minimum number of stones that needs to be recolored.

Constraints

1 \leq N \leq 2\times 10^5

S is a string of length N consisting of . and #.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the minimum number of stones that needs to be recolored.

Sample Input 1

3
#.#

Sample Output 1

1

It is enough to change the color of the first stone to white.

Sample Input 2

5
#.##.

Sample Output 2

2

Sample Input 3

9
.........

Sample Output 3

0

Solution in C++14 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main()
{
  int N;
  string S;
  int count;
  bool matched;
  
  cin >> N >> S;
  
  if (N == 1) {
    cout << 0;
    return 0;
  }
  
  if (N == 2 and S[0] == '#' and S[1] == '.') {
    cout << 1;
    return 0;
  }
  
  // replace # to .
  int numBtoW = 0;
  matched = false;
  for (int i = N-2; i >= 0; --i) {
    if (!matched and S[i] == '#' and S[i+1] == '.') {
      matched = true;
      ++numBtoW;
    }
    else if (matched and S[i] == '#')
      ++numBtoW;
    else if (matched and S[i] == '.')
      matched = false;
  }
  
  // replace . to #
  int numWtoB = 0;
  matched = false;
  for (int i = 1; i < N; ++i) {
    if (!matched and S[i-1] == '#' and S[i] == '.') {
      matched = true;
      ++numWtoB;
    }
    else if (matched and S[i] == '.')
      ++numWtoB;
    else if (matched and S[i] == '#')
      matched = false;
  }
  
  cout << min(numBtoW, numWtoB);
}