#include "iostream"
#include "climits"
#include "list"
#include "queue"
#include "stack"
#include "set"
#include "functional"
#include "algorithm"
#include "math.h"
#include "utility"
#include "string"
#include "map"
#include "unordered_map"
#include "iomanip"
#include "random"

using namespace std;
const long long int MOD = 1000000007;

long long int power(long long int x, long long int n, long long int M) {
	long long int tmp = 1;

	if (n > 0) {
		tmp = power(x, n / 2, M);
		if (n % 2 == 0) tmp = (tmp*tmp) % M;
		else tmp = (((tmp*tmp) % M)*x) % M;
	}
	return tmp;
}

long long int N, M, K, Q, W, H, L, R;
long long int ans;


int main() {
	ios::sync_with_stdio(false);
	string s;
	cin >> s;
	map<char, int>m;
	for (auto i : s) {
		m[i]++;
	}
	int cnt = 0;
	char box = 0;
	for (char i = 'a'; i <= 'm'; i++) {
		if (!m[i]) {
			cnt++;
			box = i;
		}
	}
	if (cnt == 1) {
		cout << box << endl;
		return 0;
	}
	if (!cnt) {
		for (char i = 'a'; i <= 'm'; i++) {
			cout << i << endl;
		}
		return 0;
	}
	cout << "IMPOSSIBLE\n";
	return 0;
}