#include <bits/stdc++.h>
using namespace std;
long long power(long long a, long long b) {
  long long second = 1;
  while (b--) second *= a;
  return second;
}
int32_t main() {
  double r;
  cin >> r;
  cout << 2 * acos(-1.0) * r;
  return 0;
}
