Prime no list -Input 1 should be lesser than Input 2. Both the inputs should be positive. i

package monu; import java.util.Scanner;

public class Test {

    public static void main (String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.nextLine());
        int b = Integer.parseInt(sc.nextLine());
        int c=b-a;
        if((a > b) || c<0||a <0|| b <= 0){
            System.out.println("Provide valid input");
        }
        else{
            int i = 0, num = 0;
            String prime = "";

            for(i = a;i <= b;i++){
                int counter = 0;
                num = i;
                while(num >= 1){
                    if(i % num == 0)
                        counter++;
                    num--;
                }
                if(counter == 2)
                    prime = prime + i + " ";
            }

            System.out.println(prime);
        }
    }
}

This is my code there is hidden test should have got check for zero