n, m = gets.split.map &:to_i
s = gets.strip
t = gets.strip
puts [s.chars.select { |i| i == ?A } .size, t.chars.select { |i| i == ?A } .size].min + [s.chars.select { |i| i == ?B } .size, t.chars.select { |i| i == ?B } .size].min