#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
int inf = 0x3f3f3f3f;
long double PI = 3.141592654;
int main() {
  long double r;
  cin >> r;
  long double kk = 2 * r * PI;
  printf("%.15lf\n", kk);
  return 0;
}
