#include<iostream>
#include<cstring>

using namespace std;

int main()
{
    int res = 0;
    string s1, s2;
    cin >> s1 >> s2;
    for (int i = 0; i < s1.size() - 1; i++)//每次翻动当前和它后面的一个硬币，所以到倒数第二个硬币的时候就已经翻转完成了
    {
        if (s1[i] != s2[i])
        {
            res++;
            if (s1[i] == '*') s1[i] = 'o';
            else s1[i] = '*';
            if (s1[i+1] == '*') s1[i+1] = 'o';
            else s1[i+1] = '*';
        }
    }
    cout << res << endl;
}