public static int alternate(String s) {
        if (s.length() == 1) {
            return 0;
        }
        
        int maxAlternateLength = 0;
        
        for (char c1 = 'a'; c1 <= 'z'; c1++) {
            for (char c2 = (char) (c1 + 1); c2 <= 'z'; c2++) {
                int alternateLength = 0;
                char prev = '\0';
                boolean isValid = true;
                
                for (char ch : s.toCharArray()) {
                    if (ch == c1 || ch == c2) {
                        if (ch == prev) {
                            isValid = false;
                            break;
                        }
                        alternateLength++;
                        prev = ch;
                    }
                }
                
                if (isValid) {
                    maxAlternateLength = Math.max(maxAlternateLength, alternateLength);
                }
            }
        }
        
        return maxAlternateLength;
    }