/*
 * 1.cpp
 *
 *  Created on: 2017年8月30日
 *      Author: Administrator
 */


#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
string result,s,lock;
int temp = 0 ,answer = 1e9, n;
void press(int i){
	s[i-1] = s[i-1] == '1'?'0':'1';
	s[i] = s[i] == '1'?'0':'1';
	if(i+1<n)
		s[i+1] = s[i+1] == '1'?'0':'1';
}

int main(){
	cin>>lock>>result;
	s = lock;
	n = lock.size();
	//第一種情況，當前字元不相同時，從下一個字元開始按按鈕
	for( int i = 1; i<n; i++){
		if(s[i-1] != result[i-1]){
			press(i);
			temp ++ ;
		}
	}
	//可能的結果一:
	if(s[n-1] == result[n-1]){
		answer = temp ;
	}

//	第二種情況：如果前兩個按鈕不匹配，按第一個按鈕，其他按鈕不匹配還是按後一個按鈕
	s = lock;
	temp = 1;
	if( s[0] != result[0] || s[1] != result[1]){
		s[0] = s[0] == '1'?'0':'1';
		s[1] = s[1] == '1'?'0':'1';
		for(int i = 1; i< n; i++){
			if(s[i-1] != result[i-1]){
				press(i);
				temp ++;
			}
		}
	}

	//可能的結果二:
		if(s[n-1] == result[n-1]){
			answer = min(answer,temp);
		}

	if(answer == 1e9){
		cout<< "impossible";
	}
	else{
		cout<< answer;
	}

	 return 0 ;
}