#include <iostream>
#include <string>
using namespace std;

int main() {
    string init, target;
    cin >> init >> target;
    int n = init.size();

    int count = 0;
    for (int i = 0; i < n - 1; i++) {
        if (init[i] != target[i]) {
            // Flip at position i (affects i and i+1)
            init[i] = (init[i] == '*') ? 'o' : '*';
            init[i + 1] = (init[i + 1] == '*') ? 'o' : '*';
            count++;
        }
    }

    cout << count << endl;
    return 0;
}