3.4 jmu-java-随机数-使用蒙特卡罗法计算圆周率的值 (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; //产生随机数

                y= random.nextDouble()*s;

              

                if(x*x+y*y<1) k++;

               

            }

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

            System.out.println(pi);

            sc.close();

        }

    }

以上代码可以在eclipse中运行却在PTA上显示错误。因为我不了解如何产生-1到1之间的随机数。