What is the sentinel value in the following code snippet?

public static void main$($String$[]$ args$)$

$\{$

int age $=0$;

int sumOfAges $=0$;

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$0$fAges $=$ sum$0$fAges $+$ age;

System.out.println$("$Enter an age $(-1$ to stop$)$: $")$;

age $=$ reader. nextInt $()$ ;

$\}$

System.out.println$("$Sum of ages $"+$ sumOfAges$)$;

return $0$ ;

$\}$

$2$

$1$

$-1$

$0$