#include <bits/stdc++.h>

#define MAX 1001000

char map(char c)
{
    if(c == 'A')
        return 0;
    if(c == 'C')
        return 1;
    if(c == 'G')
        return 2;

    return 3;
}

void swap(char *str, int idx1, int idx2)
{
    char h = str[idx1];
    str[idx1] = str[idx2];
    str[idx2] = h;
}

int main()
{
    char strIn[MAX];
    char strOut[MAX];
    int count;

    fgets(strIn, MAX, stdin);
    fgets(strOut, MAX, stdin);
    count = strlen(strIn) - 1;

    for(int i = count - 1; i >= 0; i--)
    {
        strIn[i] = map(strIn[i]);
        strOut[i] = map(strOut[i]);
    }

    std::vector<int> swapIdxs[4][4];
    for(int i = count - 1; i >= 0; i--)
    {
        char from = strIn[i];
        char to = strOut[i];
        swapIdxs[from][to].push_back(i);
    }


    int swaps = 0;
    for(int i = 0; i < count; i++)
    {
        //printf("%s\n%s\n\n", strIn, strOut);
        if(strIn[i] == strOut[i])
            continue;
        
        char from = strIn[i];
        char to = strOut[i];
        //printf("BBB %d %c %c\n", i, from+'0', to+'0');

        if(swapIdxs[to][from].size() > 0)
        {
            //printf("AAA\n");
            swaps++;
            int idx1 = swapIdxs[from][to].back();
            int idx2 = swapIdxs[to][from].back();
            swapIdxs[from][to].pop_back();
            swapIdxs[to][from].pop_back();
            swap(strIn, idx1, idx2);
        }
        else
        {
            for(int g = 0; g < 4; g++)
            {
                if(g == from) // dont swap A<=>A
                    continue;
                
                if(swapIdxs[to][g].size() > 0)
                {
                    swaps++;
                    int idx1 = swapIdxs[from][to].back();
                    swapIdxs[from][to].pop_back();
                    int idx2 = swapIdxs[to][g].back();
                    swapIdxs[to][g].pop_back();
                    swapIdxs[from][g].push_back(idx2);
                    swap(strIn, idx1, idx2);
                    break;
                }
            }
        }        
    }

    printf("%d", swaps);


    return 0;
}