#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  std::cerr << name << " : " << arg1 << '\n';
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  std::cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
signed main() {
  ios_base::sync_with_stdio(0);
  long long TESTS = 1;
  while (TESTS--) {
    long long r;
    cin >> r;
    cout << fixed << setprecision(10) << 2 * acos(-1) * r;
  }
}
