#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e+18 + 1;
const long long MOD = 1000000007LL;
string abc = "abcdefghijklmnopqrstuvwxyz";
string ABC = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
int main() {
  long long r;
  cin >> r;
  cout << r * 3.14159265359 << endl;
}
