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

def valid?(s)
  range = 0...s.size
  !s.size.times.any?{|i|
    case s[i]
    when ?R
      range.cover?(i - Kr) && s[i - Kr] == ?R ||\
      range.cover?(i + Kr) && s[i + Kr] == ?R
    when ?B
      range.cover?(i - Kb) && s[i - Kb] == ?B ||\
      range.cover?(i + Kb) && s[i + Kb] == ?B
    else
    end
  }
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