N = gets.to_i
S = gets.chomp
s = S.dup

checked = Hash.new
ans = 0

0.upto(N - 2) do |i|
  s[i] = 'B'
  s[i + 1] = 'B'
  hash = s.hash
  s[i] = S[i]
  s[i + 1] = S[i + 1]

  next if checked[hash]
  checked[hash] = true
  ans += 1
end

puts ans