//POJ8469特殊密码锁
#include<cstdio>
#include<iostream>
#include<cstdlib>
#include<algorithm>
#include<cstring>
using namespace std;
char s[110], s1[110], s2[110], model[110];
int main() {
int count1 = 1, count2 = 0;
cin >> s >> model;
memcpy(s1, s, sizeof(s));//s1的0位置被按下
memcpy(s2, s,sizeof(s));//s2的0位置未被按下
s1[0] = !(s1[0] - '0') + '0';
s1[1] = !(s1[1] - '0') + '0';
for(int i = 0; model[i + 1]; i++) {
if(s1[i] != model[i]) {
s1[i] = !(s1[i] - '0') + '0';
s1[i + 1] = !(s1[i + 1] - '0') + '0';
if(s1[i + 2]) s1[i + 2] = !(s1[i + 2] - '0') + '0';
count1++;
}
}
if(strcmp(s1, model)) count1 = -1;
for(int i = 0; model[i + 1]; i++) {
if(s2[i] != model[i]) {
s2[i] = !(s2[i] - '0') + '0';
s2[i + 1] = !(s2[i + 1] - '0') + '0';
if(s2[i + 2]) s2[i + 2] = !(s2[i + 2] - '0') + '0';
count2++;
}
}
if(strcmp(s2, model)) count2 = -1;
if(count1 == -1 && count2 == -1) {
printf("impossible\n");
return 0;
}
if(count1 == -1) printf("%d\n", count2);
else if(count2 == -1) printf("%d\n", count1);
else printf("%d\n", min(count1, count2));
return 0;
}
