#include <iostream>
#include <cstdlib>
#include <vector>
#include <cmath>
#include <utility>
#include <algorithm>
#include <set>
#include <queue>
#include <climits>
#include <limits>
#include <map>

#define ll long long
#define max(A, B) ((A) > (B) ? (A) : (B))
#define min(A, B) ((A) < (B) ? (A) : (B))
#define mp(A, B) make_pair(A, B)
#define pb push_back
#define fu(A, B, C) for(ll (A) = (B); (A) < (C); (A)++)
#define fu0(A, B) fu((A), 0, (B))
#define fd(A, B, C) for(ll (A) = (B); (A) >= (C); (A)--)
#define mem(A, B) memset((A), (B), sizeof((A)))
#define v(A, B, C) vector<ll> (A)((B), (C));
#define v0(A, B) v((A), (B), 0);


using namespace std;

typedef pair<int, int> pi;

void perform() {
  int n; cin >> n;
  cout << (2 * M_PI * n) << endl;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ll t = 1; //cin >> t;
  fu0(i, t) {perform();}
}
