National University of Singapore

Alternative Anagram

Consider a string of $N$ letters, where $N$ is an even number. Rearrange the letters such that all $N/2+1$ substrings of length $N/2$ are different.


A single line containing string $S$ of length $N$, where $2 \leq N \leq 10^5$ and $N$ is even. The string consists of lower-case letters from the English alphabet, “a” to “z”.


If possible, print a string of length $N$ that is a rearrangement of the letters in $S$ such that all substrings of length $N/2$ are different. If this is impossible, print $-1$.

If there is more than one solution, any one will do.

Explanation of Sample Inputs

In the first example, the substrings before rearrangement are “tral”, “rala”, “alal”, “lala”, and “alal”. Note that “alal” appears twice. After the rearrangement, all substrings are different.

In the third example, all substrings are different already in the input, so it suffices to print the original string.

Sample Input 1 Sample Output 1
Sample Input 2 Sample Output 2
Sample Input 3 Sample Output 3