#include <bits/stdc++.h>
using namespace std;
const long long LINF = 1e18L + 1;
const int INF = 1e9 + 1;
int main() {
  int R;
  cin >> R;
  double ans = R * 2 * M_PI;
  printf("%.10f\n", ans);
  return 0;
}
