#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
const int N=110;
int n;
char start[N],end2[N];
void turn(int x)
{
if(start[x]=='o')start[x]='*';
else start[x]='o';
}
int main()
{
cin>>start>>end2;
n=strlen(start);
int res=0;
for(int i=0;i<n-1;i++)
if(start[i]!=end2[i])
{
res++;
turn(i),turn(i+1);
}
cout<<res;
}