from copy import *
n = int(input())
s = list(input())
S = []
l = 0
for i in range(n-1):
    if s[i] != s[i+1]:
        S.append(s[l:i+1])
        l = i+1
if S[-1][0] == s[-1]:
    S[-1] += s[l:]
else:
    S.append(s[l:])


ans = 0
m = len(S)

if S[0][0] == '#':
    for i in range(m//2):
        ans += min(len(S[i*2]), len(S[i*2+1]))
else:
    for i in range(m//2 - 1):
        ans += min(len(S[i*2+1]), len(S[i*2+2]))

print(ans)