def remove_duplicates(s):
    # Check for invalid special characters
    for ch in s:
        if not (ch.isalnum() or ch.isspace()):
            return "Invalid input"

    seen = set()
    result = []
    for ch in s:
        if ch not in seen:
            seen.add(ch)
            result.append(ch)
    return "".join(result)

# Main
s = input().strip()
print(remove_duplicates(s))