Answer:
Written in Java Programming Language
import java.util.Scanner;
public class Assign{
 public static void main(String [] args) {
 Scanner stdin = new Scanner(System.in);
 int num,sum=0;
 System.out.println("Start Inputs: ");
 num = stdin.nextInt();
 while (num>=0) {
 if(num%2== 0)
 {
 sum+=num; 
 } 
 num = stdin.nextInt();
 }
 System.out.print("Sum of even numbers: "+sum);
 
 }
}
Step-by-step explanation:
I'll start my explanation from line 4
This line initializes Scanner object stdin
Scanner stdin = new Scanner(System.in);
This line declares necessary variables and initializes sum to 0
 int num,sum=0;
This line prompts user for inputs
 System.out.println("Start Inputs: ");
This line reads user inputs
 num = stdin.nextInt();
This line checks is user input is not negative (It's a loop that will be repeated continually until a negative number is inputted)
 while (num>=0) {
This line checks if user input is an even number
 if(num%2== 0)
 {
If the above condition is true, this line calculates the sum of even numbers
 sum+=num; 
 } 
This line prepares the user for next input
 num = stdin.nextInt();
 }
This line prints the calculated sum of even number; If no even number is inputted, the sum will be 0
 System.out.print("Sum of even numbers: "+sum);