#include <bits/stdc++.h>
using namespace std;
const double PI = 3.14159265358979323846;
const string YES = "Yes";
const string NO = "No";
void solve(long long N) {
  double result = (double)N * 2 * PI;
  printf("%8.20f\n", result);
}
int main() {
  long long N;
  scanf("%lld", &N);
  solve(N);
  return 0;
}
