#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 10;
void solve() {
  int n;
  cin >> n;
  cout << 2 * acos(-1) * n << endl;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
