#include <bits/stdc++.h>
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;
}
using Graph = vector<vector<long long>>;
long long fg(long long k) {
  long long sum = 1;
  for (long long i = 1; i <= k; ++i) {
    sum *= i;
  }
  return sum;
}
signed main() {
  long long a;
  cin >> a;
  cout << 2 * a * M_PI;
}
