#include <iostream>
#include <iostream>
#include <bits/stdc++.h>
#include <string>
#include <numeric>
#include <vector>
#include <cmath>
#include <map>
#include <ctype.h>
using namespace std;

int main() {
    double r; cin >> r;
    cout << 2.0 * M_PI * r  << endl;



}
