#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using uint = unsigned;
using ull = unsigned long long;
using P = pair<int, int>;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
const int mod = 1000000007;
int main() {
  int r;
  cin >> r;
  printf("%.4lf\n", 2 * r * 3.1415);
  return 0;
}
