#include<bits/stdc++.h> 
using namespace std;
string a,b; int len,num1,num2;
bool opta,optb; int c;
int main(){
	cin >> a >> b; num1 = num2 = -1;
	len = a.size();
	for(int i = 0;i < len;i++){
		opta = optb = 0;
		if('a' <= a[i] && a[i] <= 'z'){
			opta = true;
		}
		if('a' <= b[i] && b[i] <= 'z'){
			optb = true;
		}
		if(opta != optb){
			cout << "IMPOSSIBLE";
			return 0;
		}
		if(a[i] <= b[i]){
			c = b[i] - a[i];
		}else{
			if(opta == 1){
				c = b[i] - a[i] + 26; 
			}else{
				c = b[i] - a[i] + 10; 
			}
		}
		if(opta && num1 != -1 && num1 != c){
			cout << "IMPOSSIBLE"; return 0;
		}
		if(!opta && num2 != -1 && num2 != c){
			cout << "IMPOSSIBLE"; return 0;
		}
		if(opta && num1 == -1) num1 = c;
		if(!opta && num2 == -1) num2 = c;
	} int x = 0;
	if(num1 == -1){cout << num2; return 0;}
	if(num2 == -1){cout << num1; return 0;}
	while(1){if((x * 26 + num1 - num2) % 10 == 0){
			cout << x * 26 + num1; break;
		}x++;
	}
	return 0;
}