Code to improve

Example.java

class Sample {
    public String solution(String S) {
        int len = S.length();
        if (len >= 2 && len <= 100) {
            char[] charList = new char[len];
            charList = S.toCharArray();

            int charDash = 0;
            String sol = "";
            int count = 0;
            for (int i = 0; i < len; i++) {
                if (charList[i] != ' ' &&
                        charList[i] != '-') {
                    if (Character.getNumericValue(charList[i]) <= 9 &&
                            Character.getNumericValue(charList[i]) >= 0) {
                        if (count != 0 && count % 3
                                == 0) {
                            sol = sol + "-" +
                                    charList[i];
                        } else {
                            sol += charList[i];
                        }
                        if (charDash == 2) {
                            count = count + 2;
                            charDash=0;
                        } else {
                            count++;
                        }
                    } else {
                        return "INVALID INPUT: expected Numeric values";
                    }

                } else if (i!=0 && charList[i] ==
                        '-' && charList[i - 1] == '-') {
                    charDash = 2;
                }
            }
            return sol;
        } else {
            return "INVALID INPUT: expected atleast characters";
        }
    }
}