#include<bits/stdc++.h>
#define ll long long
#define il inline
#define pn putchar('\n')
#define maxint 2147483647
#define min(x,y) (x<y?x:y)
#define max(x,y) (x>y?x:y)
using namespace std;
int n=-1,m=-1;
string s,t;
signed main()
{
    #ifndef ONLINE_JUDGE
    freopen("in.txt","r",stdin);
    #endif
    cin>>s>>t;
    for(int i=0,len=s.size();i<len;i++)
    {
        if(s[i]>='0'&&s[i]<='9'&&t[i]>='0'&&t[i]<='9')
        {
            int tmp=t[i]-s[i];
            if(tmp<0)tmp+=10;
            if(m!=-1&&m!=tmp)
                return puts("IMPOSSIBLE"),0;
            m=tmp;
        }
        else if(s[i]>='a'&&s[i]<='z'&&t[i]>='a'&&t[i]<='z')
        {
            int tmp=t[i]-s[i];
            if(tmp<0)tmp+=26;
            if(n!=-1&&n!=tmp)
                return puts("IMPOSSIBLE"),0;
            n=tmp;
        }
        else
            return puts("IMPOSSIBLE"),0;
    }
    if(m==-1)return printf("%d",n),0;
    if(n==-1)return printf("%d",m),0;
    for(int ans=n;;ans+=26)
        if(ans%10==m)
            return printf("%d",ans),0;
    return 0;
}