#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
const double pi = acos(-1);
void solve() {
  double r;
  cin >> r;
  double ans = pi * 2 * r;
  printf("%.6f\n", ans);
}
int main() { solve(); }
