#include <iostream>
#include <string.h>
#include <algorithm>
#include <cstdio>
using namespace std;
char s1[40],s2[40],s3[40];
int c1,c2,ans1,ans2,len;

void Swap(char *s,int i)
{
    s[i-1]= s[i-1]=='1'? '0' :'1';
    s[i] = s[i]=='1'? '0' :'1';
    if(i+1 < len)
        s[i+1
 
] = s[i+1]=='1'? '0' :'1';
}

int main()
{
    while(~scanf("%s %s",s2,s3))
    {
        strcpy(s1,s2);
        len = strlen(s1);
        ans1= ans2=1e9;
        c1 = 0,c2 = 1;
        bool flag =0;
        for(int i=1;i<len;i++)
        {
            if(s1[i-1] != s3[i-1])
            {
                Swap(s1,i);
                c1
 
++;
            }
        }
        if(strcmp(s1,s3)==0)
        {
            flag =1;
            ans1=c1;
        }
        strcpy(s1,s2);
        s1[0]= s1[0]=='1'? '0' :'1';//相當於已經按了第一個了
        s1[1]= s1[1]=='1'? '0' :'1';
        for(int i=1;i<len;i++)
        {
            if(s1[i-1] != s3[i-1])
            {
                Swap(s1,i);c2
 
++;
            }
        }
        if(strcmp(s1,s3)==0)
        {
            flag =1;
            ans2 =c2;
        }
        if(flag)
        {
            printf("%d\n",min(ans1,ans2));
        }
        else
        {
            puts("impossible");
        }
    }

}