// Problem: C - Crossed out letter
// Contest: Virtual Judge - Namomo Summer Camp 23 Day 2
// URL: https://vjudge.net/contest/576885#problem/C
// Memory Limit: 254 MB
// Time Limit: 1000 ms

#include <bits/stdc++.h>
using namespace std;

// 2023 OneWan
int cnt[28];
int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	string a, b;
	cin >> a >> b;
	int n = a.size();
	if (a == b) {
		cout << a << "a";
	} else if (n == 1) {
		cout << a << b;
	} else {
		int pos;
		for (int i = 0 ; i < n ; i++) {
			if (a[i] != b[i]) {
				pos = i;
				break;
			}
		}
		string ta = a.substr(0, pos) + b[pos] + a.substr(pos);
		string tb = b.substr(0, pos) + a[pos] + b.substr(pos);
		pos = -1;
		for (int i = 0 ; i < n ; i++) {
			if (ta[i] != b[i]) {
				pos = i;
				break;
			}
		}
		if (pos == -1) {
			cout << ta;
			return 0;
		}
		string sb = b.substr(0, pos) + ta[pos] + b.substr(pos);
		if (ta == sb) {
			cout << ta;
			return 0;
		}
		pos = -1;
		for (int i = 0 ; i < n ; i++) {
			if (tb[i] != a[i]) {
				pos = i;
				break;
			}
		}
		if (pos == -1) {
			cout << tb;
			return 0;
		}
		string sa = a.substr(0, pos) + tb[pos] + a.substr(pos);
		if (tb == sa) {
			cout << tb;
			return 0;
		}
		cout << "IMPOSSIBLE";
	}
	return 0;
}