def count s,c
  if u=s[0]
    k=[H[u],count(s[1..-1],(u.ord-1).chr)].min
    (u.succ..c).inject(0){|t,r|t+H[r]}+k+(H[u]-k)/2
  else
    (?a..c).inject(0){|t,r|t+H[r]}
  end
end
gets
H=Hash[gets.chars.group_by{|c|c}.map{|k,v|[k,v.size]}]
H.default=0
p count("yuki",?z)