#include <bits/stdc++.h>
using namespace std;
string s,t;
int n,cnt;
map<string,int>mpt,mps;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	cin>>s>>t;
	n=s.size();
	for(int i=0; i<n; i++) {
		if(('a'<=s[i]&&s[i]<='z'&&'0'<=t[i]&&t[i]<='9')||('a'<=t[i]&&t[i]<='z'&&'0'<=s[i]&&s[i]<='9')) {
			cout<<"IMPOSSIBLE\n";
			return 0;
		}
	}
	while(s!=t) {
		if(mps[s]&&mpt[t]) {
			cout<<"IMPOSSIBLE\n";
			return 0;
		}
		mps[s]=1;
		mpt[t]=1;
		for(int i=0; i<n; i++) {
			if(s[i]=='z')s[i]='a';
			else if(s[i]=='9')s[i]='0';
			else s[i]++;
		}
		cnt++;
	}
	cout<<cnt<<endl;
	return 0;
}