What is the sentinel value in the following code snippet?
public static void main(String[] args)
{
int age = 0;
int sum_of_ages = 0;
int stop = 1;
Scanner reader = new Scanner(System. in);
System. out. println("Enter an age (-1 to stop): ");
age = reader. nextInt();
while (age != -1)
{
sum_of_ages = sum_of_ages + age;
System. out. println("Enter an age (-1 to stop): ");
age = reader. nextInt();
}
System. out. println("Sum of ages " + sum_of_ages);
return 0;
}