#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
const long long mod = 1e9 + 7;
const long long N = 2e5 + 9;
const long double pi = 3.14159265358979323846;
void raushan_kumar() {
  long long d = 0, e = 0, f = 0, g = 0;
  double f1 = 0.0, f2 = 0.0, f3 = 0.0;
  long long flag = 0, ans = 0, res = 0, cnt = 0, sum = 0;
  long double n;
  cin >> n;
  long double k = 2 * pi * n;
  cout << k;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) {
    raushan_kumar();
  }
}
