#include <bits/stdc++.h>
using namespace std;
int main() {
string source;
string dest;
cin >> source >> dest;
int cnt = 0;
for(int i = 0;i<source.size();i++) {
if(source[i] != dest[i]) {
if(source[i] == '*'){
source[i] = 'o';
}else {
source[i] = '*';
}
if(source[i + 1] == '*'){
source[i + 1] = 'o';
}else {
source[i + 1] = '*';
}
cnt++;
}
}
cout << cnt << endl;
}