#include <bits/stdc++.h>
using namespace std;
long long n, t, m, c1, c2, c, x, y, sum, mn = 999999999, avg, pos, dif, j, dis,
                                         mx, xi, yi, q;
vector<long long> v;
set<int> s;
map<long long, long long> mp;
bool PE(int x) {
  for (int i = 2; i * i <= x; i++)
    if (x % i == 0) return false;
  return true;
}
bool PR(string s) {
  int n = s.length();
  for (int i = 0; i < n / 2; i++)
    if (s[i] != s[n - i - 1]) return false;
  return true;
}
int main(void) {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  ;
  long double p = 6.28318530717958623200 / 2.0;
  cin >> n;
  cout << 2 * p * n;
}
