#include <bits/stdc++.h>
using namespace std;
void get(long long arr[], long long n) {
  for (long long i = 0; i < n; i++) cin >> arr[i];
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  long long r;
  cin >> r;
  float c = 2 * 3.141592 * r;
  cout << fixed << setprecision(3) << c;
}
