#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 5, mod = 1000000007;
double r;
int main() {
  cin.tie(0);
  cout.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> r;
  r *= 3.14159265;
  r *= 2;
  cout << r;
  return 0;
}
