import java.util.*

fun main() {
    q1157()
}

fun q1157() = with(Scanner(System.`in`)) {
    val input = next()
    val alphabetMap = input.uppercase(Locale.getDefault()).toCharArray().toList().groupingBy { it }.eachCount()
    val maxValue = alphabetMap.values.maxOf { it }
    val maxKeys = alphabetMap.filter { it.value == maxValue }.keys
    println(if(maxKeys.count() == 1) maxKeys.first() else "?" )
}