#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  double r;
  cin >> r;
  double circum = 2 * M_PI * r;
  printf("%.2g\n", circum);
}
