#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const long long INF = 1e12;
const int inf = 1e9;
const double PI = 3.14159265358979323846;
int main() {
  int r;
  cin >> r;
  printf("%.5lf\n", 2 * r * PI);
  return 0;
}
