Java/com/study/demo/test2_1.java

package com.study.demo; 
import java.util.Scanner;

public class test4 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double r1 = sc.nextDouble();
        double r2 = sc.nextDouble();
        sc.close();

        double R = 1 / (1 / r1 + 1 / r2);
        System.out.printf("%.2f\n", R);
    }
}