import sys
from bisect import bisect_right


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    input()
    s = input()
    t = input()
    sa, sb, ta, tb = s.count('A'), s.count('B'), t.count('A'), t.count('B')
    return min(sa, ta) + min(sb, tb)


if __name__ == '__main__':
    ret = main()
    if ret is not None:
        print(ret)