3.4 jmu-java-隨機數-使用蒙特卡羅法計算圓周率的值 (10 分)

import java.util.Scanner;

import java.util.Random;

public class Main {

        public static void main(String[] args) {

             Scanner sc = new Scanner(System.in);

            long seed=sc.nextLong();

            int n=sc.nextInt();     

             Random  random= new Random(seed);

            double  x,y=0;

           

            long k=0,i=0;

            for(i=0;i<n;i++){

                int s;

                if (i<1/n) {s=1;}

                else {s=-1;}

                x= random.nextDouble()*s; //產生隨機數

                y= random.nextDouble()*s;

              

                if(x*x+y*y<1) k++;

               

            }

            double pi=4*(double)k/(double)i;

            System.out.println(pi);

            sc.close();

        }

    }

以上代碼可以在eclipse中運行卻在PTA上顯示錯誤。因為我不了解如何產生-1到1之間的隨機數。