#include <bits/stdc++.h>
using namespace std;
long long int ncr(long long int n, long long int r) {
  if (n == 1 || r == 0) return 1;
  if (r > n / 2) r = n - r;
  long long int ans = 1;
  for (long long int i = 1; i <= r; i++) {
    ans = ((ans * n) / i);
    n--;
  }
  return ans;
}
long long int ncrsum(long long int n, long long int r, long long int mod) {
  if (n == 1 || r == 0) return 1;
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  long long int t1;
  t1 = 1;
  while (t1--) {
    long long int n, i1, i2, k, m, j, j1, f, a1, i31, i3, j2, i5, i4, i6, i7, i;
    i4 = INT_MIN, i1 = 0;
    f = 0;
    long long int res, res1;
    long long int s1, s2, s3;
    s2 = 0;
    s1 = 0;
    double pi = 3.141592653589793238;
    cin >> n;
    cout << 2 * pi * n;
  }
}
