#include <bits/stdc++.h>
using namespace std;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
__attribute__((constructor)) void initial() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}
const double PI = 3.141592;
int main() {
  double R;
  double two = 2;
  cin >> R;
  cout << (R * PI * two) << endl;
}
