import java.util.*;
 
class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
 
        String s = sc.nextLine();
 
        int[] freq = new int[26];
        Queue<Character> q = new LinkedList<>();
        StringBuilder ans = new StringBuilder();
 
        for (char ch : s.toCharArray()) {
 
            // Count frequency
            freq[ch - 'a']++;
 
            // Add current character to queue
            q.offer(ch);
 
            // Remove characters that are repeating
            while (!q.isEmpty() && freq[q.peek() - 'a'] > 1) {
                q.poll();
            }
 
            // First non-repeating character
            if (q.isEmpty()) {
                ans.append('#');
            } else {
                ans.append(q.peek());
            }
        }
 
        System.out.println(ans);
    }
}

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