#include <iostream>
#include <cstdio>
#include <string>
using namespace std;
string s,t;
bool f;
int main()
{
	f=false;
	cin>>s>>t;
	int lens=s.length();
	int lent=t.length();
	for(int i=0;i<lens;i++) 
	{
		//b2 c9
		if(s[i]>='a'&&s[i]<='z'&&t[i]>='a'&&t[i]<='z') 
			f=0; 
		else if(s[i]>='0'&&s[i]<='9'&&t[i]>='0'&&t[i]<='9') 
			f=0;
		else 
		{
			f=true;
			break;
		}
	}
	if(f==1) 
	{
		puts("IMPOSSIBLE");
		return 0;
	}
	int minn=1e9,ans=0;
	while(s!=t)
	{
		for(int i=0;i<lens;i++)
		{
			if(s[i]>='0'&&s[i]<='9')
				if(s[i]!='9') s[i]+=1; else s[i]='0';
			if(s[i]>='a'&&s[i]<='z')
				if(s[i]!='z') s[i]+=1; else s[i]='a';
		}
		ans++;
		minn=min(minn,ans);
	}
	if(minn==1e9) puts("0"); else printf("%d",ans);
	return 0;
}