#include<bits/stdc++.h>

using namespace std;

//不會 

int main(){

string a;

getline(cin,a);

int l=a.length();

string b;

bool out=false;

while(getline(cin,b)&&b.length()!=0){

if(b.length()==l){

int f=0,c[26]={0},d[26]={0};

for(int i=0;i<l;i++){

if(a[i]==b[i]){

f=1;break;

}

if(c[a[i]%26]==0)c[a[i]%26]=b[i]-'a'+1;

if(d[b[i]%26]==0)d[b[i]%26]=a[i]-'a'+1;

if(c[a[i]%26]!=b[i]-'a'+1){

f=1;break;

}

if(d[b[i]%26]!=a[i]-'a'+1){

f=1;break;

}

}

if(f==0)cout<<b<<endl,out=true;

}

b="";

}

if(!out)cout<<"impossible\n";

return 0;

}