#include <bits/stdc++.h>
using namespace std;
void perform() {
  int n;
  cin >> n;
  cout << (2 * M_PI * n) << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t = 1;
  for (long long((i)) = (0); ((i)) < ((t)); ((i))++) {
    perform();
  }
}
