#include "bits/stdc++.h"
using namespace std;
#define fastcin {\
    cin.tie(0);\
    ios::sync_with_stdio(false);\
}
#define rep(i, a, b) for(int i = a; i < b; i++)
#define rrep(i, a, b) for(int i = a; i >= b; i--)
#define fore(i, a) for(auto &i:a)
#define print(x) cout << x << "\n"
#define SORT(a, n) sort(a, a+n);
#define REVERSE(a,n) reverse(a,a+n);
#define VSORT(v) sort(v.begin(), v.end());
#define VREVERSE(v) reverse(v.begin(), v.end());
#define MOD 1000000007
#define yes "YES"
#define no "NO"
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vll;

int main() {
    fastcin;
    int cl[13] = {};
    char c;
    string a = "";
    rep(i, 0, 13) {
        cin >> c;
        if(cl[c-'a']<=1) cl[c-'a']++;
        else a = "impossible";
    }
    rep(i, 0, 13) {
        if(cl[i]==0 && a.size()==0) {
            a = 'a' + i;
        } else if(cl[i]==0 && a.size()!=0) {
            a = "impossible";
        }
    }
    if(a.size()==0) a = "abcdefghijklm";
    if(a=="impossible") print(a);
    else rep(i, 0, a.size())print(a[i]);
    return 0;
}