#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <algorithm>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>
#include <bits/stdc++.h>
using namespace std;
 
//priority_queue<long long, vector<long long>, greater<long long>> pq;

const double TL = 1.5 * CLOCKS_PER_SEC;
mt19937 rng((unsigned int) chrono::steady_clock::now().time_since_epoch().count());
#define clr(x, y) memset(x, y, sizeof(x))
#define forn(i, n) for (int i = 0; i < n; i++)
#define LL long long


const double pi = acos(-1.0);

int main() {
  //auto start_clock = clock();
#ifdef LOCAL
  freopen("in", "r", stdin);
#endif
  double r;
  cin >> r;
  cout << 2 * pi * r << endl;
  return 0;
}