#include <bits/stdc++.h>

using namespace std;

int main() {
    //读入输出优化的强迫症
    ios::sync_with_stdio(false);

    double sum = 0;
    for (int i = 0;; i++) {
        double term = 1.0 / (i * 2 + 1);
        if (i % 2 == 0)sum += term;
        else sum -= term;
        if (term < 1e-6) break; //注意科学记数法
    }
    printf("%.6f\n", sum); //保留六位小数

    //本程序耗时
    printf("Time used = %.2f\n", (double) clock() / CLOCKS_PER_SEC);
    return 0;
}