#include <bits/stdc++.h>
using namespace std;
const int MOD = 998244353;
long double PI = 4 * atan(1);
long double eps = 1e-12;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  cout << 2 * PI * n << endl;
}
