#include <bits/stdc++.h>
const long long mod = 1e9 + 7;
using namespace std;
int n;
void solve() {
  cin >> n;
  printf("%.18f", 2 * 3.14159265389793238 * n);
}
int main() {
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
