#include <bits/stdc++.h>
using namespace std;
static const long long INF = 1e18;
static const long long MAX = 101010;
static const long long MOD = 1e9 + 7;
int main() {
  long long i, j, k;
  double pi = 3.1415926;
  long long r;
  cin >> r;
  printf("%.10f\n", r * 2 * pi);
}
