#include <bits/stdc++.h>
using namespace std;
long long int gcd(long long int a, long long int b) {
  return b ? gcd(b, a % b) : a;
}
long long int lcm(long long int a, long long int b) {
  return a / gcd(a, b) * b;
}
const long long int MOD = 1e9 + 7;
const long long int INF = 1e10;
const long double PI = (acos(-1));
signed main() {
  double R;
  cin >> R;
  double ans = 2 * R * PI;
  printf("%.10f", ans);
}
