先统计每个字符的次数，再遍历到第一个次数为1的字符

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String string = in.nextLine();
        char[] charArray = string.toCharArray();
        Map<Character, Integer> linkedHashMap = new LinkedHashMap<>();
        for (int i = 0; i < charArray.length; i++) {
            char c = charArray[i];
            Integer integer = linkedHashMap.get(c);
            if (integer == null) {
                linkedHashMap.put(c, 1);
            } else {
                linkedHashMap.put(c, integer + 1);
            }
        }
        boolean ok = false;
        for (Map.Entry<Character, Integer> entry : linkedHashMap.entrySet()) {
            if (entry.getValue() == 1) {
                System.out.println(entry.getKey());
                ok = true;
                break;
            }
        }

        if (!ok) {
            System.out.println("-1");
        }
    }
}