#include <bits/stdc++.h>
using namespace std;
const int MOD1 = 1e9 + 7;
const int MOD2 = 998244353;
const int INF = INT_MAX;
const double PI = 3.1415926;
int main() {
  double n;
  cin >> n;
  cout << fixed << setprecision(5) << n * PI * 2;
}
