#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const int nax = 1e5 + 5;
int n, m, k, q;
int main() {
  {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
  };
  int circle_radius;
  float PI_VALUE = 3.14159265359, circle_area, circle_circumf;
  scanf("%d", &circle_radius);
  circle_area = PI_VALUE * circle_radius * circle_radius;
  circle_circumf = 2 * PI_VALUE * circle_radius;
  printf("%.10f", circle_circumf);
  return (0);
}
