#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
const int INF = 0x3f3f3f3f;
const double EPS = 1e-10;
const unsigned long long mod = 998244353;
const double II = 3.1415926535;
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(0), cout.tie(0);
  int r;
  scanf("%d", &r);
  double sum = 2 * r * II * 1.0;
  printf("%lf\n", sum);
}
