#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vl = vector<ll>;
using pii = pair<int, int>;
using vpii = vector<pair<int, int>>;
template <typename T>
using pque = priority_queue<T, vector<T>, greater<T>>;
int main() {
  ld r;
  scanf("%Lf", &r);
  printf("%.30Lf\n", 2 * r * M_PI);
}
