#include <iostream>
#include <cstring>
#include <string>
using namespace std;
int len, cnt = 0;
string s, t, cs;
int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	cin >> s >> t;
	if (s == t) {
		cout << "0" << "\n";
		return 0;
	}
	len = s.size();
	for (int i = 0; i < len; i ++ )
		if((s[i] >= 'a' && s[i] <= 'z' && t[i] >= '0' && t[i] <= '9') || (s[i] >= '0' && s[i] <= '9' && t[i] >= 'a' && t[i] <= 'z')) {
			cout << "IMPOSSIBLE" << "\n";
			return 0;
		}
	cs = s;
	while(true) {
		if (cs == t) {
			cout << cnt << "\n";
			return 0;
		}
		if (cnt && cs == s) {
			cout << "IMPOSSIBLE" << "\n";
			return 0;
		}
		cnt ++ ;
		for (int i = 0; i < len; i ++ ) {
			if (cs[i] == 'z') {
                cs[i] = 'a';
                continue;
            }
			if (cs[i] == '9') {
                cs[i] = '0';
                continue;
            }
			cs[i] = char(int(cs[i]) + 1);
		}
	}
	return 0;
}