#include <bits/stdc++.h>
using namespace std;
using ll = long long;
void read();
template <typename First, typename... Rest>
void read(First& first, Rest&... rest);
void write();
template <typename First, typename... Rest>
void write(First first, Rest... rest);
template <typename T>
void write_vec(vector<T>& v, char divider);
int main() {
  int R;
  read(R);
  cout << fixed << setprecision(2) << 2 * M_PI * R;
  return 0;
}
void read() {}
template <typename First, typename... Rest>
void read(First& first, Rest&... rest) {
  cin >> first;
  read(rest...);
}
void write() {}
template <typename First, typename... Rest>
void write(First first, Rest... rest) {
  cout << first << " ";
  write(rest...);
}
template <typename T>
void write_vec(vector<T>& v, char divider) {
  for (size_t i = 0; i < v.size(); i++) {
    cout << v[i] << divider;
  }
}
