package main

import (
	"fmt"
	"strings"
)

func main() {
	var s string
	fmt.Scanln(&s)
	s = strings.ToLower(s)
	var freq [26]int
	for _, r := range s {
		freq[r-'a']++
	}
	first := int(s[0] - 'a')
	freq[first]--
	for i := 1; i < len(s); i++ {
		ch := int(s[i] - 'a')
		if first < ch && freq[ch] == 1 {
			break
		}
		if ch < first && 0 < freq[first] {
			first = ch
		}
		freq[ch]--
	}
	fmt.Println(string(first + 'a'))
}