491D-2

#include <bits/stdc++.h>

using namespace std;

int main() {
    string s[2];

    int n = 2;
    int m;
    for (int i = 0; i < n; i++) {
        cin >> s[i];
        m = s[i].length();
    }
    n = m;

    int ans = 0;
    int empty = 0;
    for (int i = 0; i < n; i++) {
        int current = (s[0][i] == '0') + (s[1][i] == '0');
        empty += current;
        if (empty >= 3) {
            empty -= 3;
            ans++;
        }
        else
            empty = current;
    }

    cout << ans;
}