#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
using Graph = vector<vector<int> >;
const int MOD = 1000000007;
long long GCD(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return GCD(b, a % b);
}
int H, W;
int field[100][100];
const int dx[2] = {1, 0};
const int dy[2] = {0, 1};
const double PI = 3.14159265358979323846;
bool seen[100][100];
int main() {
  double r;
  cin >> r;
  cout << 2.0 * PI * r << endl;
}
