import sequtils, strutils, tables

let
  n = stdin.readLine.split.map parseInt
  s, t = stdin.readLine
var s_cnt, t_cnt = initCountTable[char]()

for c in s:
  s_cnt.inc c
for c in t:
  t_cnt.inc c

echo min(s_cnt['A'], t_cnt['A']) + min(s_cnt['B'], t_cnt['B'])