#include <bits/stdc++.h>
using namespace std;
using P = pair<int, int>;
long long int gcd(long long int a, long long int b) {
  if (a < b) {
    long long int tmp = a;
    a = b;
    b = tmp;
  }
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
long long int lcm(long long int a, long long int b) {
  return a * b / gcd(a, b);
}
int main() {
  int r;
  cin >> r;
  double ans = r * 3.14159265359 * 2;
  cout << ans << endl;
}
