#include <bits/stdc++.h>
using namespace std;
const int mods = 1e9 + 7;
const int maxn = 1e5 + 10;
const int N = 1e5 + 10;
const int E = 2e5 + 10;
long long n;
int main() {
  double r;
  cin >> r;
  r = acos(-1) * 2.0 * r;
  printf("%llf\n", r);
}
