#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9;
const int MOD = 1e9 + 7;
const double PI = 3.141592653589;
double ans;
int main() {
  double a, b, c;
  string s, t;
  cin >> a;
  printf("%lf\n", a * 2 * PI);
}
