#include <bits/stdc++.h>
using namespace std;
const int Max = 2e6 + 10;
const int Mod = 1e9 + 7;
const long long int Inf = 1LL << 62;
int main() {
  long double n;
  cin >> n;
  n *= 2.0;
  n *= acos(-1);
  cout << setprecision(10) << n << fixed;
  return 0;
}
