# coding: utf-8
import re

intLength = int(input())
strStones = input()

intCount = 0
strPre = ""

aryBlack = re.findall("#", strStones)
if len(aryBlack) > intLength / 2:
    strBase = "#"
    strChange = "."
else:
    strBase = "."
    strChange = "#"

for intIdx in range(intLength):
    if strPre == strBase and strStones[intIdx] == strChange:
        intCount += 1
        strPre = strBase
    else:
        strPre = strStones[intIdx]

print(intCount)