为什么我的Java扫描程序不会输入？

package Lessons;

import java.util.Scanner;

public class Math {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        System.out.print("please enter the first number of your question! ");
        int a1 = s.nextInt();
        System.out.print("So you chose " + a1);
        System.out.println(" Choose ur second number");
        int a2 = s.nextInt();
        System.out.print("You chose " + a2 + " for your second number");
        System.out.print(" Now enter ur operator ");
        String b1 = s.nextLine();
        if (b1.equalsIgnoreCase("-")) {
            System.out.println("You chose " + b1 + "as ur operator");
        }
    }

}
package Lessons;

import java.util.Scanner;

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