// Connected Components
//https://docs.google.com/document/d/1sgA5C_JzwPK8Qrip98Ignjj1NI29bqlIpmpg2SHKTTw/edit?tab=t.0
import java.io.*;
import java.util.*;
class Main {
static char[] par;
static List<Character> list;
if(s1.
length()!=s2.
length()) System.
out.
println(-1); else{
int flag=0;
for(int i=0;i<s1.length();i++){
if((int)s1.charAt(i)>(int)s2.charAt(i)){
flag=1;
break;
}
}
if(flag==0){
list=new ArrayList<>();
for(int i=0;i<s1.length();i++){
if(!list.contains(s1.charAt(i))) list.add(s1.charAt(i));
if(!list.contains(s2.charAt(i))) list.add(s2.charAt(i));
}
par=new char[list.size()];
for(int i=0;i<list.size();i++){
par[i]=list.get(i);
}
for(int i=0;i<s1.length();i++){
setPar(s2.charAt(i),s1.charAt(i));
}
int count=0, ans=0;
List<Character> con=new ArrayList<>();
for(int i=0;i<list.size();i++){
char a=findPar(list.get(i));
if(!con.contains(a)){
count=1;
for(int j=i+1;j<list.size();j++){
if(findPar(list.get(j))==a){
count++;
}
}
ans+=count-1;
con.add(a);
}
}
}
}
return;
}
static char findPar(char a){
if(par[list.indexOf(a)]==a) return a;
return findPar(par[list.indexOf(a)]);
}
static void setPar(char a, char b){
char parA=findPar(a);
par[list.indexOf(b)]=parA;
return;
}
}
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