#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
char stra[105], strb[105];
void turn(int idx)
{
stra[idx] = stra[idx] == 'o' ? '*' : 'o';
}
int main()
{
scanf("%s%s", stra, strb);
int n = strlen(stra), cnt = 0;
for (int i = 0; i < n-1; ++i)
{
if (stra[i] != strb[i])
{
turn(i), turn(i+1);
++cnt;
}
}
printf("%d", cnt);
return 0;
}