#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 7;
const long long longinf = 1LL << 60;
const long long mod = 1e9 + 7;
int main() {
  int R;
  cin >> R;
  cout << setprecision(10) << 2 * 3.1415926535 * R << endl;
  return 0;
}
