#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
const double PI = 3.14159265358979323846;
int main() {
  double r;
  cin >> r;
  double ans;
  ans = 2 * PI * r;
  printf("%.10f", ans);
  return 0;
}
