#include<iostream>
#include<string>
using namespace std;
string a,b;
void turn(int i)
{
if(a[i] == 'o') a[i] = '*';
else a[i] = 'o';
}
int main()
{
cin >> a >> b;
int cnt = 0;
for(int i = 0; i < a.size()-1;i++)
if(a[i] != b[i]) cnt++, turn(i), turn(i+1);
cout << cnt << endl;
return 0;
}