#include <bits/stdc++.h>
using namespace std;
const int N = 2 * 1e5 + 5;
const long long mod = 998244353;
const int INF = INT_MAX;
const long long INFCAD = long long(INT_MAX) * 2 + 1;
const double E = 1e-9;
int main() {
  int r;
  cin >> r;
  cout << fixed << setprecision(12) << 2.0 * acos(-1) * r << '\n';
  return 0;
}
