#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using mp = map<char, int>;
const int MOD = 1e9 + 7;
const int INF = 1001001001;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int r;
  cin >> r;
  cout << r * 2 * 3.141592 << "\n";
  return 0;
}
