#include <bits/stdc++.h>
using namespace std;
void dump(vector<int>& v) {
  for (int x : v) {
    cout << x << endl;
  }
}
int gcd(int a, int b) {
  if (a % b == 0) {
    return (b);
  } else {
    return (gcd(b, a % b));
  }
}
int lcm(int a, int b) { return a * b / gcd(a, b); }
int combination(int a, int b) { return combination(a - 1, b); }
int main(void) {
  int n, ans;
  cin >> n;
  ans = (2 * n) * 3.14159265359;
  cout << ans << endl;
  return 0;
}
