牛牛学数列3

#include <iostream>

using namespace std;

int main()
{
    int num;
    cin >> num;

    double sum = 0;
    const double numerator = 1.0;  // 分子
    for (int i = 1; i <= num; i++) {
        int flag = 1;
        double denominator = 0; // 分母
        for (int j = 1; j <= i; j++) {
            denominator += (2 * j - 1) * flag;
            flag = -1 * flag;
        }
        sum += numerator / denominator;
    }
    printf("%.3lf", sum);
    return 0;
}