#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<vector>
#include<string>
#include<map>


using namespace std;

char arr1[1005];
char arr2[1005];

void op(int n)//函数操作两个字符串
{
	if (arr2[n] == '*') arr2[n] = 'o';
	else arr2[n] = '*';
	if (arr2[n+1] == '*') arr2[n+1] = 'o';
	else arr2[n+1] = '*';
}

int main(void)
{
	cin.getline(arr1, 1000).getline(arr2, 1000);
//数据输入
	int len = strlen(arr1);
	int ans = 0;
	for (int i = 0; i < len; i++)
	{
		if (arr1[i] != arr2[i])
		{
			ans++;
			op(i);
		}

	}
	cout << ans;
//思想就是模拟翻硬币的过程
	return 0;
}