I take Apply online assessement through Codesignal. I did not pass 5 hidden test cases.

Question:
Given a string word as input, return the minimum number of characters added into the word to become anagram - become a palindrome.

For example, if word = "apple", it's possible to add two letters 'a' and 'e', which makes the resulting string an anadrome: "appleae" letters can be rearranged to become "apelepa", which is a palindrome.

Here is my solution below, could you guys help point out which part is incorrect?

	public int minimum(String s){
		
		int[] letters = new int[26];
		for(char c : letters.toCharArray()){
			letters[c-'a']++;
		}
		
		int res = 0;
		int skip = 1; // 1 or 3 repeat characters do not need to add extra one character
		
		for(int cnt : letters){
			if(cnt > 3 && cnt%2==1){
				res++;
			}else if(cnt <= 3 && cnt%2 ==1){
				if(skip==1){
					skip--;
					continue;
				}
				res++;
			}
		}
		
		return res;
	}