N, M = gets.split.map(&:to_i)
S = gets.chomp
T = gets.chomp

counter_a = S.chars.tally
counter_b = T.chars.tally
counter_a.default = 0
counter_b.default = 0

puts [counter_a['A'], counter_b['A']].min + [counter_a['B'], counter_b['B']].min