#include <bits/stdc++.h>
using namespace std;
template <class A, class B>
inline bool chmax(A &a, const B &b) {
  return b > a && (a = b, true);
}
template <class A, class B>
inline bool chmin(A &a, const B &b) {
  return b < a && (a = b, true);
}
const int INF = 1 << 30;
const int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
const int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};
int main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  double r;
  cin >> r;
  cout << (r + r) * 3.141592 << '\n';
  return (0);
}
