#include <bits/stdc++.h>
using namespace std;
void solve(long long ind) {
  long long x;
  cin >> x;
  double ans = (double)22 / (double)7;
  ans = ans * 2 * (double)x;
  cout << ans;
}
int main() {
  long long t = 1, i = 1;
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  std::cout.precision(6);
  std::cout.setf(std::ios::fixed, std::ios::floatfield);
  while (i <= t) {
    solve(i);
    i++;
  }
  return 0;
}
