#include <bits/stdc++.h>

#include <iostream>
//#include <algorithm>
// #include <iomanip>
#define ll long long
#define map unordered_map
#define set unordered_set

using namespace std;

const ll MOD = 1000000007;
const ll INF = (1LL << 62);

int main() {
  std::cout << std::fixed << std::setprecision(10);

  ll a;
  scanf("%lld", &a);
  cout << (a * 2 * 3.1415926535) << endl;
}
