#include <bits/stdc++.h>
using namespace std;
long double n, x, y, a[300000], b[300000], c = 0, t = 0, sum = 0, z = 0, i = 0,
                                           j, f = 0;
char ch[101], cr, ss[101];
string s;
int gcd(int, int);
int main() {
  cin >> n;
  printf("%.12lf", 2 * acos(0) * 2 * n);
  return 0;
}
