import java.util.*;
/**
【程序20】 题目：有一分数序列：2/1，3/2，5/3，8/5，13/8，21/13...求出这个数列的前20项之和。
1.程序分析：请抓住分子与分母的变化规律。
*/

public class TestFractionSum {
    private static double sum = 0.0;

    public static void main(String args[]) {
        int temp;
        double numerator, denominator, fun;
        System.out.println("你想计算前多少项的和,请输入一个正整数：");
        Scanner input = new Scanner(System.in);
        temp = input.nextInt();
        numerator = 2.0;
        denominator = 1.0;
        for (int i = 1; i <= temp; i++) {
            
            sum = sum + numerator / denominator;
            fun = numerator + denominator;
            denominator = numerator;
            numerator = fun;
        }
        System.out.println("前" + temp + "项的和是：" + sum);
    }
}