#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using P = pair<int, int>;
using Pl = pair<ll, ll>;
int main() {
  int n;
  cin >> n;
  cout << (3.1415926535) * n * 2.0 << endl;
  return 0;
}
