#include <bits/stdc++.h>
using namespace std;
const int MAXN = (int)((1e5) + 10);
int gcd(int a, int b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
int max(int a, int b) {
  if (a > b)
    return a;
  else
    return b;
}
int min(int a, int b) {
  if (a < b)
    return a;
  else
    return b;
}
bool isPrime(int N) {
  for (int i = 2; i * i <= N; ++i) {
    if (N % i == 0) return false;
  }
  return true;
}
int cbrt(int x) {
  int lo = 1, hi = min(2000000ll, x);
  while (hi - lo > 1) {
    int mid = (lo + hi) / 2;
    if (mid * mid * mid < x) {
      lo = mid;
    } else
      hi = mid;
  }
  if (hi * hi * hi <= x)
    return hi;
  else
    return lo;
}
bool compare(const pair<int, int> &a, const pair<int, int> &b) {
  return (a.second < b.second);
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  int r;
  cin >> r;
  double d = 2 * (3.14) * r;
  cout << d << "\n";
  return 0;
}
