代码：
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
int n;
Scanner scanner = new Scanner(System.in);
n = scanner.nextInt();
double sum=0;
for(int i=0;i<n;i++)
{
sum+=Math.pow(-1,i)*1/(i*3+1);
}
System.out.println(sum);
}
}
0