#include <bits/stdc++.h>
using namespace std;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
const int long long MOD = 1e9 + 7;
static const int MAX = 100;
static const int INF = (1 << 23);
template <class T>
T gcd(T a, T b) {
  return b ? gcd(b, a % b) : a;
}
template <class T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
int main() {
  cout << setprecision(16);
  double r;
  cin >> r;
  cout << 2 * r * M_PI << endl;
  return 0;
}
