#include <bits/stdc++.h>

using namespace std;
using ll = long long;

#define REP(i, n) for (ll i = 0; i < n; i++)
#define INF 9223372036854775807
#define all(x) (x).begin(), (x).end()

ll ts = 1000000007;
const double PI = acos(-1);

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  ll r;
  cin >> r;

  cout << 2 * PI * r << "\n";
}