#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
struct edge {
  int from;
  int to;
  int cost;
};
unsigned euclidean_gcd(unsigned a, unsigned b) {
  if (a < b) return euclidean_gcd(b, a);
  unsigned r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
int main() {
  double r;
  cin >> r;
  double ans = 2 * M_PI * r;
  cout << fixed << setprecision(4) << ans << endl;
  return 0;
}
