#include <bits/stdc++.h>
using namespace std;
long long mypow(long long fa, long long fb, long long fc) {
  if (fb == 0) return 1;
  if (fb % 2) return mypow(fa, fb - 1, fc) * fa % fc;
  long long z = mypow(fa, fb / 2, fc);
  return z * z % fc;
}
long long k(long long fa) {
  long long x = 0;
  while (fa) {
    x++;
    fa /= 10;
  }
  return x;
}
long long gcd(long long fa, long long fb) {
  if (fb == 0) return fa;
  return gcd(fb, fa % fb);
}
signed main() {
  double a;
  cin >> a;
  printf("%.100lf", a * 3.14159265359);
}
