#include <bits/stdc++.h>
using namespace std;
using namespace std::chrono;
void pr_init() {}
void solve() {
  string str[2];
  cin >> str[0] >> str[1];
  long long ss = (long long)str[0].size();
  long long dp[2][ss + 1];
  for (long long i = 0; i < 2; i++) {
    for (long long j = 0; j < ss; j++) {
      if (str[i][j] == 'X')
        dp[i][j] = 0;
      else
        dp[i][j] = 1;
    }
  }
  dp[0][ss] = dp[1][ss] = 0;
  long long ways = 0;
  for (long long j = 0; j < ss; j++) {
    if (dp[0][j] == 1 && dp[1][j] == 1) {
      if (dp[0][j + 1] == 1)
        ways++, dp[0][j + 1] = 0;
      else if (dp[1][j + 1] == 1)
        ways++, dp[1][j + 1] = 0;
      continue;
    } else if (dp[0][j] == 1) {
      if (dp[0][j + 1] == 1 && dp[1][j + 1] == 1)
        ways++, dp[0][j + 1] = 0, dp[1][j + 1] = 0;
      continue;
    } else if (dp[1][j] == 1) {
      if (dp[0][j + 1] == 1 && dp[1][j + 1] == 1)
        ways++, dp[0][j + 1] = 0, dp[1][j + 1] = 0;
      continue;
    }
  }
  cout << ways << endl;
}
int32_t main() {
  pr_init();
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  solve();
}
