#include<bits/stdc++.h>
using namespace std;
const int N=5e6+5;char a[N],b[N];
int main()
{
    scanf("%s%s",a+1,b+1);
    int len=strlen(a+1);
    for(int i=1;i<=len;i++)
    {
        if(a[i]==b[i])a[i]=b[i]=0;
        else a[i]=b[i]=1;
    }
    if(len==1)
    {
        if(a[1]==0)printf("0\n");else printf("1\n");
        return 0;
    }
    if(len==2)
    {
        if(a[1]!=a[2])printf("impossible\n");
		else if(a[1]==a[2])
		{
			if(a[1]==0)printf("0\n");else printf("1\n");
		}
        return 0;
    }
    int n1=1,n2=0;
    a[1]^=1;a[2]^=1;
    for(int i=1;i<=len-1;i++)
    {
        if(a[i]==1){a[i]^=1;a[i+1]^=1;a[i+2]^=1;n1++;}
		if(b[i]==1){b[i]^=1;b[i+1]^=1;b[i+2]^=1;n2++;}
    }
    if(a[len]==1)n1=1e8;
    if(b[len]==1)n2=1e8;
    int ans=min(n1,n2);
    if(ans==int(1e8))printf("impossible\n");
    else printf("%d\n",ans);
    return 0;
}