#include <bits/stdc++.h>
using namespace std;
void fastio() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
}
int main() {
  fastio();
  long long int T;
  T = 1;
  while (T--) {
    long long int n;
    double pi = 3.14159265358979323846;
    cin >> n;
    double ans = (double)2 * pi * double(n);
    cout << ans << endl;
  }
  return 0;
}
