#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long t, n, m, k, q, x, a, b, c, d;
  cin >> a;
  cout << (double)2 * acos(-1) * a << '\n';
}
