#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for(ll i=0;i<n;i++)
#define all(n) n.begin(),n.end()

int main() {
  float n,ans;
  cin >> n;
  ans = 2 * n * 3.141;
  cout << ans << endl;
  return 0;
}
