#include <bits/stdc++.h>
using namespace std;
signed long long int a[1000005], b[1000005], c[1000005], k = 0;
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  signed long long int n, m, f = 0;
  double pi = 3.14;
  cin >> n;
  cout << 2 * pi * n;
}
