#include <bits/stdc++.h>
using namespace std;
double pi = 3.14159265358979323846;
void sync() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
int main() {
  sync();
  long long int t;
  t = 1;
  for (long long int testcase = 1; testcase <= t; testcase++) {
    long long int r;
    cin >> r;
    double area = 2 * pi * r;
    cout << fixed << setprecision(20) << area << endl;
  }
}
