#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
const int MOD = 1000000007;
const int INF = 999999999;
int main() {
  int r;
  cin >> r;
  cout << 2 * r * acos(-1) << endl;
  return 0;
}
