#include <bits/stdc++.h>
#pragma GCC optimize("Ofast", 3, "inline")
using namespace std;
int mon[20] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
long long gcd(long long x, long long y) {
  return ((y == 0) ? x : gcd(y, x % y));
}
const double PI = 3.1415926535897932384626;
double r, C;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> r;
  C = r * 2 * PI;
  printf("%.15lf", C);
  return 0;
}
