#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
const int maxn = 1e5 + 10;
long long gcd(long long a, long long b) { return (b == 0) ? a : gcd(b, a % b); }
bool isPrime(long long n) {
  if (n <= 3) {
    return n > 1;
  }
  if (n % 6 != 1 && n % 6 != 5) {
    return false;
  }
  long long sqr = sqrt(n);
  for (long long i = 5; i <= sqr; i += 6) {
    if (n % i == 0 || n % (i + 2) == 0) {
      return false;
    }
  }
  return true;
}
int dis[4][2] = {0, 1, 0, -1, 1, 0, -1, 0};
int read() {
  int x = 0, f = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    x = (x << 1) + (x << 3) + (c ^ 48), c = getchar();
  return x * f;
}
int main() {
  int n;
  cin >> n;
  float pi = 3.1415926535898;
  printf("%lf\n", pi * 2 * n);
}
