import java.util.Scanner;

public class PalindromChecker {
	
	public void check(String str) {
		StackOfChars stack = new StackOfChars(str.length());
		
		int step = 1;
		boolean flag = true;
		
		for(int i = 0; i < str.length(); i++) {
			char s = str.charAt(i);
			if(step == 1) {
				if(s == 'a' || s == 'b') stack.push(s);
				else if(s == '@') step = 2;
				else{
					System.out.println("Error: "+s+" at "+(i+1));
					flag = false;
					break;
				}
			}//if
			else { // step = 2
				if(stack.isEmpty()) {
					System.out.println("Error: missing symbols before @");
					flag = false;
					break;
				}
				else if(s != 'a' && s != 'b') {
					System.out.println("Error: "+s+" at "+(i+1));
					flag = false;
					break;
				}
				else {
					char c = stack.pop();
					if(c != s) {
						System.out.println("Error: "+s+" at "+(i+1));
						flag = false;
						break;
					}
				}
			}//else
		}//for
			
		if(flag && step == 1) {
			System.out.println("Error: missing symbol @");
			flag = false;
		}
						
		if(flag) {
			if(!stack.isEmpty())
				System.out.println("Error: missing symbols after @");
			else System.out.println("Yes");	
		}
	}//method
}//class

class TestPalindromChecker {
	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		System.out.println("To stop testing enter: No more");
		System.out.println("Starting...");
		while(true) {
			System.out.print("     Word: ");
			String word = s.nextLine();
			if(word.equals("No more")) {
				System.out.println("End of test!");
				break;
			}
			else (new PalindromChecker()).check(word);
		}	
	}
}