#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1000005;
const int SQRTN = 1003;
const int LOGN = 22;
const double PI = acos(-1);
const int INF = 1e9;
int MOD = 1000000007;
const int FMOD = 998244353;
const double eps = 1e-9;
long long gcd(long long a, long long b) {
  if (a == 0) return (b);
  return (gcd(b % a, a));
}
long long lcm(long long a, long long b) { return (a / gcd(a, b) * b); }
void solve() {
  long long n;
  cin >> n;
  double pi = 3.141592653589793238;
  cout << fixed << setprecision(16) << double(2 * pi * n) << "\n";
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) {
    solve();
  }
  return (0);
}
