#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
constexpr int MOD = 1000000007;
constexpr int INF = 2147483647;
void yes(bool expr) { cout << (expr ? "Yes" : "No") << "\n"; }
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int R;
  cin >> R;
  cout << std::setprecision(10) << R * 3.1415926535 << endl;
}
