#include <bits/stdc++.h>
using namespace std;
long long m = 1e9 + 7;
vector<long long> fact, inv_fact;
void fast() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
long long bin_pow(long long a, long long b) {
  if (b == 0) return 1;
  long long res = bin_pow(a, b / 2);
  res = (res * res) % m;
  if (b % 2 == 1) {
    res = (res * a) % m;
  }
  return res;
}
long long inv(long long a) { return bin_pow(a, m - 2); }
long long C(long long n, long long k) {
  return (((fact[n] * inv_fact[k]) % m) * inv_fact[n - k]) % m;
}
int32_t main() {
  fast();
  double n;
  cin >> n;
  cout << 6.28318530717958623200 * n << "\n";
  return 0;
}
