#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int INF = 1e9;
const int N = 1e6 + 6;
long long powmod(long long a, long long b, long long mod) {
  long long res = 1;
  a %= mod;
  for (; b; b >>= 1) {
    if (b & 1) res = (res * a) % mod;
    a = (a * a) % mod;
  }
  return res;
}
long long gcd(long long a, long long b) {
  while (b > 0) {
    long long t = a % b;
    a = b, b = t;
  }
  return a;
}
long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }
long long is_prime(long long n) {
  if (n <= 1 || n > 3 && (n % 2 == 0 || n % 3 == 0)) return 0;
  for (long long i = 5, t = 2; i * i <= n; i += t, t = 6 - t)
    if (n % i == 0) return 0;
  return 1;
}
int main() {
  string g;
  vector<string> s;
  long long ans = 0;
  for (long long i = 0; i < 2; i++) cin >> g, s.push_back(g);
  for (long long i = 0; i < s[0].length(); i++) {
    if (i == 0) {
      if (s[0][i] == '0' && s[1][i] == '0' && s[1][i + 1] == '0') {
        ans++;
        s[0][i] = 'x';
        s[1][i] = 'x';
        s[1][i + 1] = 'x';
      } else if (s[0][i] == '0' && s[1][i] == '0' && s[0][i + 1] == '0') {
        ans++;
        s[0][i] = 'x';
        s[1][i] = 'x';
        s[0][i + 1] = 'x';
      }
    } else {
      if (s[0][i] == '0' && s[1][i] == '0' && s[1][i - 1] == '0') {
        ans++;
        s[0][i] = 'x';
        s[1][i] = 'x';
        s[1][i - 1] = 'x';
      } else if (s[0][i] == '0' && s[1][i] == '0' && s[0][i - 1] == '0') {
        ans++;
        s[0][i] = 'x';
        s[1][i] = 'x';
        s[0][i - 1] = 'x';
      } else if (s[0][i] == '0' && s[1][i] == '0' && s[1][i + 1] == '0') {
        ans++;
        s[0][i] = 'x';
        s[1][i] = 'x';
        s[1][i + 1] = 'x';
      } else if (s[0][i] == '0' && s[1][i] == '0' && s[0][i + 1] == '0') {
        ans++;
        s[0][i] = 'x';
        s[1][i] = 'x';
        s[0][i + 1] = 'x';
      }
    }
  }
  cout << ans << "\n";
  return 0;
}
