#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int MAXN = 1e5 + 3;
const int inf = 2e9;
int max(int a, int b) {
  if (a > b)
    return a;
  else
    return b;
}
int min(int a, int b) {
  if (a < b)
    return a;
  else
    return b;
}
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, -1, 1};
bool cmp(pair<int, int> p1, pair<int, int> p2) {
  if (p1.first < p2.first)
    return true;
  else if (p1.first == p2.first) {
    if (p1.second < p2.second) return true;
    return false;
  }
  return false;
}
void solve() {
  double n;
  cin >> n;
  double tpi = 6.28318530717958623200;
  cout << tpi * n << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  std::cout << std::setprecision(20);
  std::cout << std::fixed;
  int t;
  t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
