#include <bits/stdc++.h>
#pragma optimise GCC(-O2)
using namespace std;
mt19937_64 rang(
    chrono::high_resolution_clock::now().time_since_epoch().count());
int rng(int lim) {
  uniform_int_distribution<int> uid(0, lim - 1);
  return uid(rang);
}
long long INF = LLONG_MAX;
const long long M = 1000000007;
long long powm(long long, long long);
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long r;
  cin >> r;
  double pi = 3.14159265359;
  cout << setprecision(18) << 2 * pi * r;
}
long long powm(long long a, long long b) {
  long long res = 1;
  while (b) {
    if (b & 1) res = (res * a) % M;
    a = (a * a) % M;
    b >>= 1;
  }
  return res;
}
