#include<iostream>
#include<cstring>

using namespace std;

string s, e;
void turn(int x){
    s[x] = s[x] == 'o' ? '*' : 'o';
    if(x + 1 < s.size())
        s[x+1] = s[x+1] == 'o' ? '*' : 'o';
}
int main(){
    cin >> s >> e;
    int step = 0;
    for(int i = 0; i < s.size(); i++){
        if(s[i] != e[i]){
            step++;
            turn(i);
        }
    }
    cout << step;
    return 0;
}