#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 7;
const long long INFL = 9 * 1e18;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
template <typename T>
void print(T&& x) {
  cout << x << endl;
}
long long inline digit(long long num) {
  int tmp = 0;
  while (num) {
    tmp++;
    num /= 10;
  }
  return tmp;
}
template <typename T>
inline T digitSum(T num) {
  T sum = 0;
  while (num) {
    sum += num % 10;
    num /= 10;
  }
  return sum;
}
template <typename T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <typename T>
inline T lcm(T a, T b) {
  T g = gcd(a, b);
  return a / g * b;
}
template <typename T>
inline T power(T a, T b) {
  T tmp = 1;
  for (int i = 0; (i) < ((int)(b)); ++(i)) {
    tmp *= a;
  }
  return tmp;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
int ctoi(char c) { return c - '0'; }
int main() {
  int n;
  cin >> n;
  printf("%.6lf\n", n * 2 * 3.1415926535897932384626433832795028841);
  return 0;
}
