#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
const int maxn = 2e6 + 10;
const double eps = 1e-1;
int T, n, m, k, q;
int main() {
  double r, pi = acos(-1);
  scanf("%lf", &r);
  printf("%f\n", 2.0 * r * pi);
  return 0;
}
