Java Sum of numbers until string is entered

i've just started java programming and was wondering on how to approach or solve this problem i'm faced with.

I have to write a program that asks a user for a number and continually sums the numbers inputted and print the result. This program stops when the user enters "END"

I just can't seem to think of a solution to this problem, any help or guidance throughout this problem would be much appreciated and would really help me understand problems like this. This is the best i could do

public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);

while (true) {
    System.out.print("Enter a number: ");
    int x = scan.nextInt();
    System.out.print("Enter a number: ");
    int y = scan.nextInt();

    int sum = x + y;


    System.out.println("Sum is now: " + sum);   

}   


}
    }   

The output is supposed to look like this:

Enter a number: 5

Sum is now: 5

Enter a number: 10

Sum is now: 15

Enter a number: END