#include <bits/stdc++.h>
using namespace std;
vector<long long> prime;
bool isPrime(long long n) {
  if (n <= 1) return false;
  if (n <= 3) return true;
  if (n % 2 == 0 || n % 3 == 0) return false;
  for (long long i = 5; i * i <= n; i = i + 2)
    if (n % i == 0) return false;
  return true;
}
long long gcd(long long a, long long b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
long long lcm(long long a, long long b) {
  long long g = gcd(a, b);
  long long ans = (a * b) / g;
  return ans;
}
void primes(long long m) {
  bool isprime[m];
  for (int i = 0; i <= m; i++) isprime[i] = true;
  for (int i = 3; i * i <= m; i++) {
    if (isprime[i]) {
      for (int j = i * i; j <= m; j += i) isprime[j] = false;
    }
  }
  prime.push_back(2);
  for (int i = 3; i <= m; i += 2)
    if (isprime[i]) prime.push_back(i);
}
int main(int argc, char *argv[]) {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  double r;
  cin >> r;
  cout << setprecision(23);
  cout << 2 * 2 * acos(0) * r << endl;
  return 0;
}
