import strutils, sequtils, algorithm

let
  tmp = map(split readLine stdin, parseInt)
  N, M = (tmp[0], tmp[1])
  S, T = sorted(readLine stdin, cmp)
var
  i, cnt = 0

while S[i] == 'A' and T[i] == 'A':
  cnt += 1
  i += 1

i = 1
while S[^i] == 'B' and T[^i] == 'B':
  cnt += 1
  i += 1

echo cnt