import std.algorithm;
import std.array;
import std.conv;
import std.math;
import std.stdio;
import std.string;
import std.range;

int readint() {
    return readln.chomp.to!int;
}

int[] readints() {
    return readln.split.map!(to!int).array;
}

bool calc(string s) {
    int found = 0;
    foreach (c; "abcdefghijklm") {
        int[char] m;

        foreach (c2; s.filter!(e => 'a' <= e && e <= 'm'))
            m[cast(char) c2]++;
        m[c]++;

        auto n1 = m.values.count!(e => e == 1);
        auto n2 = m.values.count!(e => e == 2);
        if (n1 == 12 && n2 == 1) {
            writeln(c);
            found++;
        }
    }
    return found > 0;
}

void main() {
    auto s = readln.chomp;

    bool found = calc(s);
    if (!found)
        writeln("Impossible");
}