#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const int MAX = 10000000;
template <typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T>
T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int main() {
  double n;
  cin >> n;
  double x = 2 * n * acos(-1.0);
  printf("%0.10lf\n", x);
  return 0;
}
