#include <bits/stdc++.h>
using namespace std;
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;
  }
}
