#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
void show(int a[], int arraysize) {
  for (int i = 0; i < arraysize; ++i) {
    cout << a[i] << " ";
  }
  cout << endl;
}
int gcd(int a, int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
void sieve(int n) {
  bool prime[n + 1];
  memset(prime, true, sizeof(prime));
  for (int p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (int i = p * p; i <= n; i += p) prime[i] = false;
    }
  }
  for (int p = 2; p <= n; p++)
    if (prime[p]) cout << p << " ";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  double n;
  cin >> n;
  cout << fixed << setprecision(5) << 2 * 3.1415926535 * n << endl;
  return 0;
}
