#include <bits/stdc++.h>
using namespace std;
const long long int inf = 2000000;
long long int i = 0, T = 0, N = 0, tc = 0, t = 0;
long long int n = 0, k = 0;
string s;
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(20) << ((double)2) * acos(-1) * r;
}
