#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
bool cm(pair<string, int> p1, pair<string, int> p2) {
  return p1.second > p2.second;
}
long long par[100005], n, m;
long long find(long long a) {
  if (par[a] < 0) {
    return a;
  } else
    return par[a] = find((par[a]));
}
void xunion(long long a, long long b) {
  par[a] = min(par[a], par[b]);
  par[b] = a;
}
int32_t main() {
  long long r;
  cin >> r;
  double x = 2 * r * 3.141592653589793238;
  cout << x << "\n";
  return 0;
}
