#include <bits/stdc++.h>
using namespace std;
long long M = (long long)1e9 + 1, t = 0;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long digits(long long n) { return floor(log10(n)) + 1; }
bool ispoweroftwo(long long x) { return x && (!(x & (x - 1))); }
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(4);
  long long n;
  cin >> n;
  cout << 2 * (M_PI)*n << endl;
}
