#include <bits/stdc++.h>
using namespace std;
long long n, m, a[200005];
int main() {
  long long int i, j, c = 0, t, l, r, mid, z = 0, k, x, y;
  double d;
  scanf("%lf", &d);
  d = 2 * acos(-1.0) * d;
  printf("%.7f\n", d);
}
