#include <bits/stdc++.h>
using namespace std;
const int M = 1e5 + 5;
const int N = 1e5 + 5;
const double PI = acos(-1.0);
int m, n;
int main() {
  scanf("%d", &n);
  printf("%.4lf\n", 2 * PI * n);
  return 0;
}
