#include <bits/stdc++.h>
using namespace std;
long long power(long long a, long long b, long long m = 1000000007) {
  if (b == 0) return 1;
  if (b == 1) return a;
  long long res = power(a, b / 2, m);
  res = (res * res) % m;
  if (b & 1) res = (res * a) % m;
  return res;
}
long long modinv(long long a) { return power(a, 1000000007 - 2); }
void solve() {
  double pi = 3.14159;
  long long n;
  cin >> n;
  cout << 2 * pi * n << "\n";
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t = 1;
  while (t--) solve();
  return 0;
}
