#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main()
{

    ifstream a;
    ofstream b;
    string first,twice;
    int kol = -2;
    a.open("keyboard.in");
    a>>first>>twice;
    a.close();
    int ch [first.length()];
    for(int i = 0;i<first.length();i++){
        if(first[i]!=twice[i]&&ch[i-1]!=1){
           ch[i]=1;
           kol++;
        }
    }
    b.open("keyboard.out");
    b<<kol;
    b<<endl;
    b.close();
    return 0;
}