#include <bits/stdc++.h>
using namespace std;
const long long LINF = 1LL << 60;
const int INF = 1001001001;
const int MOD = 1000000007;
int main() {
  int r;
  cin >> r;
  cout << 2 * 3.141592 * r << endl;
  return 0;
}
