#!/usr/bin/env python3


import collections


def main():
    _ = input()
    ctr_s = collections.Counter(input())
    ctr_t = collections.Counter(input())
    res = min(ctr_s["A"], ctr_t["A"]) + min(ctr_s["B"], ctr_t["B"])
    print(res)


if __name__ == '__main__':
    main()