#include <bits/stdc++.h>
using namespace std;
const long long INF = 100100100100;
const long long MOD = 1000000007;
long long combination(long long n, long long r) {
  if (n < r) return 0;
  if (r == 0 || r == n)
    return 1;
  else if (r == 1)
    return n;
  return (combination(n - 1, r - 1) + combination(n - 1, r));
}
bool checkKaibun(string str) {
  string checkStr = str;
  reverse(checkStr.begin(), checkStr.end());
  if (str == checkStr)
    return true;
  else
    return false;
}
int keta(long long num) {
  int digits = 0;
  while (num > 0) {
    num /= 10;
    digits++;
  }
  return digits;
}
long long mpow(long long x, long long n) {
  long long ans = 1;
  while (n != 0) {
    if (n & 1) ans = ans * x % MOD;
    x = x * x % MOD;
    n = n >> 1;
  }
  return ans;
}
int gcd(int a, int b) {
  if (a % b == 0)
    return b;
  else
    return (gcd(b, a % b));
}
int main() {
  int R;
  cin >> R;
  cout << std::setprecision(6) << R * 2 * 3.1415926535 << endl;
}
