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 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>
#include <string.h>

int main(void) {
    int N;
    scanf("%d", &N);

    char S[N], O[N];
    scanf("%s", S);

    strcpy(O, S);

    int b = 0, w = 0;

    for (int i = 0; i < N; i++) {
        if (S[i] == '#') {
            if (S[i + 1] == '.') {
                S[i + 1] = '#';
                b++;
            }
        }
    }

    for (int i = N; i > 0; i--) {
        if (S[i] == '.') {
            if (S[i - 1] == '#') {
                S[i - 1] = '#';
                b++;
            }
        }
    }

    for (int i = 0; i < N; i++) {
        if (O[i] == '#') {
            if (O[i + 1] == '.') {
                O[i] = '.';
                w++;
            }
        }
    }

    for (int i = N; i > 0; i--) {
        if (O[i] == '.') {
            if (O[i - 1] == '#') {
                O[i - 1] = '.';
                b++;
            }
        }
    }

    if (b < w) {
        printf("%d\n", b);
    } else {
        printf("%d\n", w);
    }

    return 0;
}