#include <cstdio>
#include <cstring>
#include <iostream>
#include <cstdlib>
using namespace std;

#define MAXN (35)
char s[MAXN], t[MAXN], s2[MAXN];
int cnt = 0;

void an_s(int i)
{
    ++ cnt;
    if(s[i-1] == '0') s[i-1] = '1';
    else s[i-1] = '0';
    if(s[i] == '0') s[i] = '1'
 
;
    else s[i] = '0';
    if(s[i+1] == '0') s[i+1] = '1';
    else s[i+1] = '0';
}
void an_t(int i)
{
    ++ cnt;
    if(t[i-1] == '0') t[i-1] = '1';
    else t[i-1] = '0';
    if(t[i] == '0') t[i] = '1';
    else t[i] = '0';
    if(t[i+1] == '0') t[i+1] = '1';
    else t[i+1] = '0';
}

#define INF (1e9)
int
 
 main()
{
    scanf("%s", s+1);
    scanf("%s", s2+1);
    int n = strlen(s+1);
    int ans = INF;
    for(int i = 1; i <= n; ++ i) t[i] = s[i];
    for(int i = 2; i <= n; ++ i)
    {
        if(s[i-1] == s2[i-1]) continue;
        an_s(i);
    }
    if(s[n] != s2[n]) ans = INF;
    else ans = cnt;

    cnt = 0
 
; 
    an_t(1);
    for(int i = 2; i <= n; ++ i)
    {
        if(t[i-1] == s2[i-1]) continue;
        an_t(i);
    }
    if(t[n] != s2[n]) cnt = INF;
    ans = min(ans, cnt);
    if(ans >= INF) puts("impossible");
    else cout << ans << endl;
    return 0;
}

/*
0101
1010
*/