#include <bits/stdc++.h>
using namespace std;
mt19937 Rnd(chrono::high_resolution_clock::now().time_since_epoch().count());
template <typename T>
inline void chkmax(T &x, T y) {
  if (x < y) x = y;
}
template <typename T>
inline void chkmin(T &x, T y) {
  if (x > y) x = y;
}
inline int read() {
  int x = 0;
  char c = getchar();
  while (c < 48) c = getchar();
  while (c > 47) x = x * 10 + (c ^ 48), c = getchar();
  return x;
}
int main() {
  double x;
  cin >> x;
  double tmp = acos(-1);
  cout << 2.0 * tmp * x << endl;
  return 0;
}
