#include <bits/stdc++.h>
using namespace std;
void fun(long long n, map<long long, long long>& m1) {
  long long c = 0;
  while (n % 2 == 0) {
    if (m1.find(2) == m1.end()) {
      m1[2] = 1;
    } else {
      m1[2]++;
    }
    n /= 2;
  }
  for (long long i = 3; i <= sqrt(n); i++) {
    while (n % i == 0) {
      if (m1.find(i) == m1.end()) {
        m1[i] = 1;
      } else {
        m1[i]++;
      }
      n /= i;
    }
  }
  if (n > 1) {
    if (m1.find(n) == m1.end())
      m1[n] = 1;
    else
      m1[n]++;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long r;
  cin >> r;
  cout << float(2 * 22 * r) / float(7) << '\n';
}
