#include <bits/stdc++.h>
using namespace std;
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;
}
const int INF = INT_MAX;
const long long LLINF = 1LL << 60;
template <typename T_char>
char ToUpper(char cX) {
  return toupper(cX);
}
long long int digit(long long int a) {
  long long int count = 0;
  for (long long int i = 0; i < a; i++) {
    if (a == 0) {
      break;
    } else {
      a = a / 10;
      count++;
    }
  }
  return count;
}
uintmax_t comb(unsigned int n, unsigned int r) {
  if (r * 2 > n) r = n - r;
  uintmax_t dividend = 1;
  uintmax_t divisor = 1;
  for (unsigned int i = 1; i <= r; ++i) {
    dividend *= (n - i + 1);
    divisor *= i;
  }
  return dividend / divisor;
}
long long int gcd(long long int a, long long int b) {
  return b ? gcd(b, a % b) : a;
}
long long int lcm(long long int a, long long int b) {
  return a / gcd(a, b) * b;
}
int main(void) {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double r, PI = 3.1415926535897932;
  cin >> r;
  cout << 2 * r * PI << endl;
  return 0;
}
