#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long double r;
  cin >> r;
  long double pii = 3.141592653589793238;
  long double ans = 2 * pii * r;
  cout << ans << "\n";
}
