#include <bits/stdc++.h>
using namespace std;
const long long int LINF = 2000000000000000000ll;
const int INF = 1000000100;
const long long int MOD = 1e9 + 7;
int main() {
  int r;
  cin >> r;
  double PI = 3.14159265358979323846264338327950288419716939937510;
  double ans = (double)r * 2 * PI;
  printf("%.10f", ans);
  return 0;
}
