#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const long long INF = 1e9 + 7;
int main() {
  double r;
  cin >> r;
  double x = 3.14159265;
  cout << fixed << setprecision(3);
  cout << 2 * x << endl;
  return 0;
}
