#include <bits/stdc++.h>
#include <numeric>
#define rep(i,n) for (int i = 0; i < n; ++i)
#define rep1(i,n) for (int i = 1; i <= n; ++i)
template<typename T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;
const long long MOD = 1e9+7;
template<typename T> inline void add(T& a, T b) { a += b; if (a >= MOD) a -= MOD; }

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  long double r;
  cin >> r;
  long double ans = r * (long double)2 * 3.14159265358979323846;
  cout << std::fixed << std::setprecision(20) << ans << endl;
}
