using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        ReadLine();
        var s = ReadLine();
        var t = ReadLine();
        var sa = s.Count(si => si == 'A');
        var ta = t.Count(ti => ti == 'A');
        WriteLine(Math.Min(sa, ta) + Math.Min(s.Length - sa, t.Length - ta));
    }
}