#include<bits/stdc++.h>
using namespace std;
string s,ss,t;
long long ans=0;
void f(){
	ans++;
	for(int i=0;i<s.size();i++){
		if(s[i]=='z') s[i]='a';
		else if(s[i]=='9') s[i]='0';
		else s[i]++;
	}
}
int main(){
	cin>>s>>t;
	ss=s;
	if(s==t){
		cout<<0;
		return 0;
	}
	if(s.size()!=t.size()){
		cout<<"IMPOSSIBLE";
		return 0;
	}
	for(int i=0;i<s.size();i++){
		if('0'<=s[i]&&s[i]<='9'&&'a'<=t[i]&&t[i]<='z'){
			cout<<"IMPOSSIBLE";
			return 0;
		}
		if('0'<=t[i]&&t[i]<='9'&&'a'<=s[i]&&s[i]<='z'){
			cout<<"IMPOSSIBLE";
			return 0;
		}
	}
	while(true){
		f();
		if(s==ss){
			cout<<"IMPOSSIBLE";
			return 0;
		}else if(s==t){
			cout<<ans;
			return 0;
		}
	}
	return 0;
}