import java.util.Scanner;

public class HelloWorld {

	    public static void main(String[] args) throws Exception {

	    	Scanner sc = new Scanner(System.in);
	    	char[] char_array = sc.nextLine().toCharArray();

	    	sc.close();
	    	if (char_array[0] != 'a') {
	    		System.out.println("ato" + char_array[0]);
	    	}else {
	    	for (int i = 1; i < 26; i ++) {
	    		if (char_array[i] != char_array[i - 1] + 1) {
	    			System.out.println((char)(char_array[i - 1] + 1) +"to" + char_array[i]);
	    			break;
	    		}
	    	}}



	    	}

}