#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
const int N = 200005;
const long double EPS = 1e-9;
const long double PI = acos(-1.);
int main() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  int r;
  cin >> r;
  printf("%.4Lf\n", PI * r * 2);
  return 0;
}
