#include <bits/stdc++.h>

using namespace std;

string a, b;
int ans;

int main() {
    cin >> a >> b;
    for (int i = 0; i < a.size() - 1; ++i) {
        if (a[i] != b[i]) {
            a[i] = b[i];
            a[i + 1] = a[i + 1] == 'o' ? '*' : 'o';
            ans++;
        }
    }
    cout << ans << endl;
    return 0;
}