#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int NAX = 2e5 + 5, MOD = 1000000007;
class Solution {
 private:
 public:
  Solution() {}
  ~Solution() {}
  void solveCase() {
    int n;
    cin >> n;
    cout << fixed << setprecision(9);
    cout << acos(-1) * 2 * n;
  }
};
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int t = 1;
  Solution mySolver;
  for (int i = 1; i <= t; ++i) {
    mySolver.solveCase();
  }
  return 0;
}
