#include <bits/stdc++.h>
using namespace std;
const int c_YET = -1;
const int INF = 1e9 + 1;
const long long LLINF = 1e17 + 1;
const int DX[9] = {0, 0, 1, -1, 1, 1, -1, -1, 0};
const int DY[9] = {1, -1, 0, 0, 1, -1, 1, -1, 0};
const long long MOD = 1e9 + 7;
const long long MAX = 1e9;
const double PI = 3.14159265358979323846264338327950288;
int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(30);
  double R;
  cin >> R;
  cout << 2 * R * PI;
}
