#include <iostream>
#include <string>
using namespace std ;
string s1 , s2 ;
int ans = 0 , n ;
int main () {
	getline ( cin , s1 ) , getline ( cin , s2 ) ;
	n = s1.size() ;
	for ( int i = 0 ; i < n ; i ++ ) {
		if ( s1[i] != s2[i] ) {
			if ( s1[i] == 'o' ) s1[i] = '*' ;
			else s1[i] = 'o' ;
			if ( s1[i+1] == 'o' ) s1[i+1] = '*' ;
			else s1[i+1] = 'o' ;
			ans ++ ;
		}
	}
	cout << ans ;
	return 0 ;
}