#include <bits/stdc++.h>
using namespace std;
constexpr long long int TEN(int n) { return n == 0 ? 1 : 10 * TEN(n - 1); }
const int pi = 3.1415926;
int main(void) {
  double r;
  cin >> r;
  cout << 2 * r * pi << endl;
  return 0;
}
