#include <bits/stdc++.h>
using namespace std;
void sort2array(double *a, long *b, long n) {
  for (int i = 0; i < n; i++) {
    b[i] = i;
  }
  sort(b, b + n, [a](long i, long j) -> bool { return a[i] < a[j]; });
}
int wtover(int a, int b) {
  if (a % b > 0)
    return (a / b) + 1;
  else
    return a / b;
}
const int INF = 100000001, ID_MAX = 20;
int bi_e[ID_MAX] = {0};
int bi_max(long n) {
  int m = 0;
  for (m; pow(2, m) <= n; m++)
    ;
  m = m - 1;
  return m;
}
void bi_exs(long n) {
  for (int i = 0; i < (int)(ID_MAX); i++) {
    bi_e[i] = 0;
  }
  if (n < pow(2, ID_MAX)) {
    int m = bi_max(n);
    long l;
    for (m; m > -1; m--) {
      l = pow(2, m);
      bi_e[m] = n / l;
      n = n % l;
    }
  }
}
int main() {
  int i, r;
  cin >> r;
  cout << 2 * r * 3.14159265359;
}
