#include <bits/stdc++.h>
using namespace std;
long long int sum(long long int a) {
  long long int sum = 0;
  while (a > 0) {
    sum = sum + (a % 10);
    a = a / 10;
  }
  return sum;
}
long long int count_digit(long long int n) {
  long long int count = 0;
  while (n > 0) {
    n = n / 10;
    count++;
  }
  return count;
}
long long int binarySearch(long long int x, long long int y, long long int z,
                           vector<long long int> v) {
  long long int low = x;
  long long int high = y;
  long long int mid = x + (y - x) / 2;
  while (low <= high) {
    if (v[mid] == z) return mid;
    if (v[mid] < z) return binarySearch(mid + 1, high, z, v);
    if (v[mid] > z) return binarySearch(low, mid - 1, z, v);
  }
  return -1;
}
long long int modularExponentiation(long long int x, long long int y,
                                    long long int m) {
  if (y == 0) return 1;
  long long int p = modularExponentiation(x, y / 2, m) % m;
  p = (p * p) % m;
  return (y % 2 == 0) ? p : (x * p) % m;
}
long long int binaryExponentiation(long long int x, long long int n) {
  if (n == 0)
    return 1;
  else if (n % 2 == 0)
    return binaryExponentiation(x * x, n / 2);
  else
    return x * binaryExponentiation(x * x, (n - 1) / 2);
}
set<long long int> s;
void genrate(long long int n, long long int len, long long int max) {
  if (len > max) return;
  s.insert(n);
  genrate(n * 10 + 1, len + 1, max);
  genrate(n * 10 + 0, len + 1, max);
}
long long int my_lower(vector<long long int> &v, long long int X) {
  long long int lo = 0, hi = v.size() - 1;
  if (v.size() == 0) return -1;
  while (hi - lo > 1) {
    long long int mi = (lo + hi) / 2;
    if (v[mi] < X) {
      lo = mi;
    } else
      hi = mi - 1;
  }
  long long int ans = -1;
  for (long long int i = lo; i <= hi; i++) {
    if (v[i] < X) {
      ans = i;
    }
  }
  if (ans != -1 && v[ans] < X) {
    return ans;
  } else
    return -1;
}
long long int my_upper(vector<long long int> &v, long long int X) {
  if (v.size() == 0) return -1;
  long long int lo = 0, hi = v.size() - 1;
  while (hi - lo > 1) {
    long long int mi = (lo + hi) / 2;
    if (v[mi] < X) {
      lo = mi + 1;
    } else
      hi = mi;
  }
  long long int ans = -1;
  for (long long int i = hi; i >= lo; i--) {
    if (v[i] > X) {
      ans = i;
    }
  }
  if (ans != -1 && v[ans] > X) {
    return ans;
  } else
    return -1;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int tests = 1;
  while (tests--) {
    double r;
    cin >> r;
    double ans = 2 * 22 * r / 7;
    printf("%.11f", ans);
  }
}
