#include <bits/stdc++.h>/*{{{*/
using namespace std;
#define F(i, a, n) for(int i=a,i##end=n; i<i##end; i++)
#define FF(i, a, n) for(int i=n-1,i##end=a; i>=i##end; i--)
#define ll long long
#define pi pair<int, int>
#define f first
#define s second
#define vi vector<int>
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define mp make_pair
#define ar array
template<class T> bool ckmin(T &x, T y) { return x>y?x=y,true : false; }
template<class T> bool ckmax(T &x, T y) { return x<y?x=y,true : false; }
const char nl='\n';

#define ADD(a, b) (((a)=((a)+(b))%mod)<0?(a)+=mod:(a))
#define MUL(a, b) (((a)=1ll*(a)*(b)%mod)<0?(a)+=mod:(a))
#define EACH(x, a) for(auto &x : a)
#define vt vector
#define sz(x) (int)(x).size()
#define tcT template<class T
tcT> void read(vt<T> &v);
tcT> void read(T &x) { cin >> x; }
int read() { int x; read(x); return x; }
void read(double &x) { string t; read(t); x=stod(t); }
void read(long double &x) { string t; read(t); x=stold(t); }
tcT, class U> void read(pair<T, U> &x) { T a; U b; read(a); read(b); x=make_pair(a, b); }
tcT, size_t S> void read(ar<T, S> &a) { F(i, 0, S) read(a[i]); }
tcT, class... A> void read(T &t, A&... a) { read(t); read(a...); }
tcT> void read(vt<T> &x) { EACH(a, x) read(a); }

string to_string(char c) { return string(1, c); }
string to_string(bool b) { return b?"true":"false"; }
string to_string(const char* s) { return string(s); }
string to_string(string s) { return s; }
tcT> string to_string(T v) {
    bool f=1; string res="";
    EACH(x, v) { if(!f) res+=' '; f=0; res+=to_string(x); }
    res+="";
    return res;
}

tcT> void write(T x) { cout << to_string(x); }
template<class H, class... T> void write(const H &h, const T&... t) {
    write(h); write(t...);
}
void print() { write("\n"); }
template<class H, class... T> void print(const H &h, const T&... t) {
    write(h); if(sizeof... (t)) write(' '); print(t...);
}
void DBG() { cerr << "]" << endl; }
template<class H, class... T> void DBG(H h, T... t) {
    cerr << to_string(h); if(sizeof... (t)) cerr << ", "; DBG(t...);
}
#ifdef LOCAL
#define dbg(...) cerr << "LINE(" << __LINE__ << ") -> [" << #__VA_ARGS__ << "]: [", DBG(__VA_ARGS__)
#else 
#define dbg(...) 0
#endif
/*}}}*/

const int nax=300*1007;
string s0, s1;
int main() {
    ios::sync_with_stdio(0); cin.tie(0);
    read(s0, s1);
    int n=s0.size();
    if(s0==s1) {
        print(s0+"a");
    } else {
        int st=-1, en=-1;
        F(i, 0, n) if(s0[i]!=s1[i]) { st=i; break; }
        FF(i, 0, n) if(s0[i]!=s1[i]) { en=i; break; }
        int ok=1;
        F(i, st, en) ok&=s0[i]==s1[i+1];
        if(ok) { print(s0.substr(0, st)+s1[st]+s0.substr(st)); return 0;}
        // dbg(st, en);
        ok=1;
        F(i, st, en) ok&=s1[i]==s0[i+1];
        if(ok) { print(s1.substr(0, st)+s0[st]+s1.substr(st)); return 0;}
        else print("IMPOSSIBLE");
    }
}

// ===================== Notes ===================== //
// - A + B = total; ANS = A <==> ANS = total - B
// - Check array bounds!!!
// - Int overflow! especially in binary search!!
// - Consider special cases!
// - BREAK or CONTINUE or RETURN affect the input? 
// - Write stuff down and do something instead of nothing 
// - Don't get stuck on one approach
// - Check whether a set is already in another set before merging in UF prob.