#include <bits/stdc++.h>
using namespace std;
const long long mxN = 1e5 + 5;
const long long INF = 1e18;
const long long mod = 1e9 + 7;
signed main() {
  long long r;
  cin >> r;
  r = r * 2.0;
  r = r * 3.14159;
  cout << r;
}
