Kr, Kb = gets.split.take(2).map(&:to_i)
S = gets.chomp

def ng_at(s, i)
  r = []
  range = 0...s.size
  !case s[i]
  when ?R
    r << i - Kr if range.cover?(i - Kr) && s[i - Kr] == ?R
    r << i + Kr if range.cover?(i + Kr) && s[i + Kr] == ?R
  when ?B
    r << i - Kb if range.cover?(i - Kb) && s[i - Kb] == ?B
    r << i + Kb if range.cover?(i + Kb) && s[i + Kb] == ?B
  else
  end
  r
end

def valid?(s)
  !s.size.times.any?{|i|
    !ng_at(s, i).empty?
  }
end

def count_ng(s)
  count = 0
  s.size.times{|i|
    count += ng_at(s, i).size
  }
  count
end

q = {S=>1}
while !q.empty?
  qq = {}
  q.keys.each{|s|
    s.size.times{|i|
      next if s[i] == ?W
      t = s[0...i] + s[i+1..-1]
      if valid?(t)
        p t.size
        exit
      end
      qq[t] = 1
    }
  }
  q = qq
end