#include <bits/stdc++.h>
using namespace std;
using ll = long long;
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;
}
const int inf = 1000000007;
const int MOD = 1000000007;
const long long INF = 1000000000000000007;
int R;
int main() {
  cin >> R;
  cout << 3.1415926535 * double(R) * double(2) << endl;
}
