#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
long long int gcd(long long int a, long long int b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
long long int MOD(long long int n) { return ((n % mod) + mod) % mod; }
long long int power(long long int a, long long int b) {
  if (b == 0) return 1LL;
  long long int second = power(a, b / 2LL);
  second *= second;
  second %= mod;
  if (b & 1LL) return (second * a) % mod;
  return second;
}
long long int inv(long long int a) { return power(a, mod - 2); }
long long int nCr(long long int n, long long int r) {
  if (r > n) return 0LL;
  if (n - r < r) r = n - r;
  long long int ans = 1;
  for (long long int i = 0; i < r; i++) {
    ans *= (n - i);
    ans = MOD(ans);
    ans *= inv(i + 1);
    ans = MOD(ans);
  }
  return ans;
}
const long long int pi = 3.14159265358979323846;
signed main() {
  long long int r;
  cin >> r;
  cout << (2 * pi * r) << "\n";
  return 0;
}
