#include<bits/stdc++.h>
using namespace std;
int main(){
string source;
string target;
cin>>source>>target;
int sum=0;
for (int i=0;source!=target;i++){
if(source[i]!=target[i]){
source[i]=target[i];//更换紧挨着两个
if(source[i+1]=='*')source[i+1]='o';
else source[i+1]='*';
sum++;
}
}
cout<<sum;
return 0;
}