source_code: import java.util.*;
  
  public class Main {
  	static int mod = 1000000007;
    static int size = 200000;
  	static long[] fac = new long[size];
  	static long[] finv = new long[size];
  	static long[] inv = new long[size];
  	static int INF = Integer.MAX_VALUE;
  
   	public static void main(String[] args){
  		Scanner scanner = new Scanner(System.in);
  		String[] s = new String[2];
  		for(int i = 0; i < 2; i++){
  			s[i] = scanner.next();
  		}
  		int n = s[0].length();
  		char[][] c = new char[2][n];
  		for(int i = 0; i < 2; i++){
  			for(int j = 0; j < n; j++){
  				c[i][j] = s[i].charAt(j);
  			}
  		}
  		int count = 0;
  		for(int i = 0; i < n-1; i++){
  			if(c[0][i] == '0' && c[1][i] == '0' && c[0][i+1] == '0'){
  				c[0][i] = 'X';
  				c[1][i] = 'X';
  				c[0][i+1] = 'X';
  				count++;
  			}
  			if(c[0][i] == '0' && c[1][i] == '0' && c[1][i+1] == '0'){
  				c[0][i] = 'X';
  				c[1][i] = 'X';
  				c[1][i+1] = 'X';
  				count++;
  			}
  			if(c[0][i] == '0' && c[0][i+1] == '0' && c[1][i+1] == '0'){
  				c[0][i] = 'X';
  				c[0][i+1] = 'X';
  				c[1][i+1] = 'X';
  				count++;
  			}
  			if(c[0][i+1] == '0' && c[1][i+1] == '0' && c[1][i] == '0'){
  				c[1][i] = 'X';
  				c[0][i+1] = 'X';
  				c[1][i+1] = 'X';
  				count++;
  			}
  		}
  		System.out.println(count);
  	}
  	public static boolean isPrime(int n){
  		if(n == 1) return false;
  		if(n == 2 || n == 3) return true;
  		for(int i = 2; i <= Math.sqrt(n); i++){
  			if(n % i == 0) return false;
  		}
  		return true;
  	}
  	// tar の方が数字が大きいかどうか
  	static boolean compare(String tar, String src) {
  		if (src == null) return true;
  		if (src.length() == tar.length()) {
  			int len = tar.length();
  			for (int i = 0; i < len; i++) {
  				if (src.charAt(i) > tar.charAt(i)) {
  					return false;
  				} else if (src.charAt(i) < tar.charAt(i)) {
  					return true;
  				}
  			}
  			return tar.compareTo(src) > 0 ? true : false;
  		} else if (src.length() < tar.length()) {
  			return true;
  		} else if (src.length() > tar.length()) {
  			return false;
  		}
  		return false;
  	}
  	public static class Edge{
  		int to;
  		Edge(int to){
  			this.to = to;
  		}
  	}
  	public static void swap(long a, long b){
  		long tmp = 0;
  		if(a > b){
  			tmp = a;
  			a = b;
  			b = tmp;
  		}
  	}
    static class Pair implements Comparable<Pair>{
      int first, second;
      Pair(int a, int b){
          first = a;
          second = b;
      }
      @Override
      public boolean equals(Object o){
          if (this == o) return true;
          if (!(o instanceof Pair)) return false;
          Pair p = (Pair) o;
          return first == p.first && second == p.second;
      }
      @Override
      public int compareTo(Pair p){
          return first == p.first ? second - p.second : first - p.first; //firstで昇順にソート
          //return (first == p.first ? second - p.second : first - p.first) * -1; //firstで降順にソート
          //return second == p.second ? first - p.first : second - p.second;//secondで昇順にソート
          //return (second == p.second ? first - p.first : second - p.second)*-1;//secondで降順にソート
      }
    }
  
    //繰り返し二乗法
    public static long pow(long x, long n){
      long ans = 1;
      while(n > 0){
        if((n & 1) == 1){
          ans = ans * x;
          ans %= mod;
        }
        x = x * x % mod;
        n >>= 1;
      }
      return ans;
    }
  
  	public static long div(long x, long y){
  		return (x*pow(y, mod-2))%mod;
  	}
  
    //fac, inv, finvテーブルの初期化、これ使う場合はinitComb()で初期化必要
  	public static  void initComb(){
  		fac[0] = finv[0] = inv[0] = fac[1] = finv[1] = inv[1] = 1;
  		for (int i = 2; i < size; ++i) {
  			fac[i] = fac[i - 1] * i % mod;
  			inv[i] = mod - (mod / i) * inv[(int) (mod % i)] % mod;
  			finv[i] = finv[i - 1] * inv[i] % mod;
  		}
  	}
  
  	//nCk % mod
  	public static long comb(int n, int k){
  		return fac[n] * finv[k] % mod * finv[n - k] % mod;
  	}
  
  	//n! % mod
  	public static long fact(int n){
  		return fac[n];
  	}
  
  	//(n!)^-1 with % mod
  	public static long finv(int n){
  		return finv[n];
  	}
  
    static class UnionFind {
      int[] parent;
      public UnionFind(int size) {
        parent = new int[size];
        Arrays.fill(parent, -1);
      }
      public boolean unite(int x, int y) {
        x = root(x);
        y = root(y);
        if (x != y) {
          if (parent[y] < parent[x]) {
            int tmp = y;
            y = x;
            x = tmp;
          }
          parent[x] += parent[y];
          parent[y] = x;
          return true;
        }
        return false;
      }
      public boolean same(int x, int y) {
        return root(x) == root(y);
      }
      public int root(int x) {
        return parent[x] < 0 ? x : (parent[x] = root(parent[x]));
      }
      public int size(int x) {
        return -parent[root(x)];
      }
    }
  	public static int upperBound(int[] array, int value) {
  			 int low = 0;
  			 int high = array.length;
  			 int mid;
  			 while( low < high ) {
  					 mid = ((high - low) >>> 1) + low; // (high + low) / 2
  					 if( array[mid] <= value ) {
  							 low = mid + 1;
  					 } else {
  							 high = mid;
  					 }
  			 }
  			 return low;
  	 }
  	 public static final int lowerBound(final int[] arr, final int value) {
      	int low = 0;
      	int high = arr.length;
      	int mid;
      	while (low < high){
          	mid = ((high - low) >>> 1) + low;    //(low + high) / 2 (オーバーフロー対策)
          	if (arr[mid] < value) {
            	low = mid + 1;
          	} else {
              high = mid;
          	}
      	}
      	return low;
  		}
    //n,mの最大公約数
    public static long gcd(long n, long m){
      if(m > n) return gcd(m,n);
      if(m == 0) return n;
      return gcd(m, n%m);
    }
  	//3要素のソート
  	private class Pair2 implements Comparable<Pair2> {
  		String s;
  		int p;
  		int index;
  		public Pair2(String s, int p, int index) {
  				this.s = s;
  				this.p = p;
  				this.index = index;
  		}
  
  		public int compareTo(Pair2 other) {
  				if (s.equals(other.s)) {
  						return other.p - this.p;
  				}
  				return this.s.compareTo(other.s);
  		}
  	}
  	//c -> intに変換
  public static int c2i(char c){
  	if('A' <= c && c <= 'Z'){
  		return c - 'A';
  	}else{
  		return c - 'a' + 26;
  	}
  }
  // int -> charに変換
  public static char i2c(int i){
  	if(0 <= i && i < 26){
  			return (char)(i + 'A');
  	}else{
  			return (char)(i + 'a' - 26);
  	}
  }
  
  }
  