import Foundation
let rdArr = Array(readLine()!)
var ordArr = Array(NSOrderedSet(array: rdArr))
var num = 0
let lowChaAry = (97...122).map{String(Character(UnicodeScalar($0)!))}

for i in 0 ..< ordArr.count {
    if ordArr[i] as! String != lowChaAry[i] {
        num = i
        break
    }
}
if num == 0 {
    print("None")
} else {
    print(lowChaAry[num])
}