import std.stdio, std.string, std.conv, std.algorithm, std.numeric;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;


void main() {
    string s;
    scan(s);
    
    auto cnt = new int[](26);

    foreach (ch ; s) {
        cnt[ch - 'a']++;
    }

    int one = -1;

    foreach (i ; 0 .. 26) {
        if (cnt[i] > 2) {
            writeln("Impossible");
            return;
        }
        else if (cnt[i] == 1) {
            if (one != -1) {
                writeln("Impossible");
                return;
            }
            else {
                one = i;
            }
        }
    }

    writeln(('a' + one).to!char);
}



void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}



void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}