#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using i64 = int64_t;
const int N = 1e4 + 5;
ll read() {
  char c = getchar();
  while (!isdigit(c)) c = getchar();
  int num = 0;
  while (isdigit(c)) num = num * 10 + c - '0', c = getchar();
  return num;
}
double pi = 3.1605;
ll _;
double x;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> _;
  x = (2 * _) * pi;
  cout << fixed << setprecision(20) << x;
  return 0;
}
