How To Use While Loop To Read Values

So I'm trying to write a program that takes in 20 numbers and computes their average and I'm supposed to use a while loop to read the values and to make sure that all the numbers are positive. My while loop is set up and does all the math correctly but I have completely forgotten how to taken 20 numbers from the user all at once and then run them through the while loop.

HERE IS MY CODE SO FAR:

public static void main(String[] args) {
int num = 100;
int count = 0;
int total = 0;
while (num >= 0 && count < 20){
if (num %2 == 0){
total += num;
count++;
}
}
double avg = total/20;
System.out.println("The Result Is: " + avg);
}