1 #include <iostream>
 2 #include <cstdio>
 3 #include <string>
 4 using namespace std;
 5 
 6 string s;
 7 int n, cnt[26];
 8 
 9 bool check(int x)
10 {
11     for (int i = 0; i < 26; i++)
12     {
13         if (cnt[i] > x / 2 + (x & 1))
14         {
15             return false;
16         }
17     }
18     return true;
19 }
20 
21 int main()
22 {
23     cin >> s;
24     n = s.length();
25     int m = n;
26     for (int i = 0; i < n; i++)
27     {
28         cnt[s[i] - ‘a‘]++;
29     }
30     if (!check(n))
31     {
32         puts("INVALID");
33         return 0;
34     }
35     int last = -1;
36     for (int i = 0; i < n; i++)
37     {
38         for (int j = 0; j < 26; j++)
39         {
40             if (cnt[j] && j != last)
41             {
42                 cnt[j]--;
43                 if (check(m - 1))
44                 {
45                     putchar(‘a‘ + j);
46                     last = j;
47                     m--;
48                     break;
49                 }
50                 else
51                     cnt[j]++;
52             }
53         }
54     }
55     puts("");
56     return 0;
57 }