#include <bits/stdc++.h>
using namespace std;
const long long int INF = 1e6;
const long long int inf = 1e9 + 7;
vector<pair<long long int, long long int> > p;
long long int gcd(long long int a, long long int b) {
  if (a % b == 0)
    return (b);
  else
    return (gcd(b, a % b));
}
long long int keta(long long int N) {
  int tmp{};
  while (N > 0) {
    tmp += (N % 10);
    N /= 10;
  }
  N = tmp;
  return N;
}
int main() {
  long long int r;
  cin >> r;
  cout << 2 * r * 3.141592 << endl;
}
