let baseChars = "abcdefghijklm".map({$0})

let inpt = readLine()!.map({$0})
let imp = "Impossible"
var isImp = false;
var dic = [Character:Int]()
for c in inpt {
    if(!baseChars.contains(c)) {
        isImp = true
        break
    }
    dic[c] = inpt.filter({$0 == c}).count
    if(dic[c]! > 2) {
        isImp = true
        break
    }
}

if(!isImp) {
    if(dic.count == baseChars.count) {
        for c in baseChars {
            print(c)
        }
    } else {
        let tmp = baseChars.filter({!inpt.contains($0)})[0]
        print(tmp)
    }
} else {
    print(imp)
}