n, m = gets.chomp.split.map &:to_i
s = gets.chomp.chars.sort.group_by{|i| i }
t = gets.chomp.chars.sort.group_by{|i| i }

a = [ (s["A"] || []).size, (t["A"] || []).size ].min
b = [ (s["B"] || []).size, (t["B"] || []).size ].min

p a + b