import Foundation

extension Array {
    func unique() -> [Self.Element] {
        return NSOrderedSet(array: self).array as! [Self.Element]
    }
}
let b = readLine()!
let rdArr = Array(b/*readLine()!*/).sorted()
var ordArr = rdArr.unique()
var num = -1
let lowChaAry = (97...122).map{String.Element(extendedGraphemeClusterLiteral: Character(UnicodeScalar($0)!))}

for i in 0 ..< ordArr.count {
//    print(lowChaAry[i])
//    print(ordArr[i])
    if ordArr[i] != lowChaAry[i] {
        num = i
        break
    }
}

if num == -1 {
    print("None")
} else {
    print(lowChaAry[num])
}