#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization("unroll-loops")
using namespace std;
const double pi = acos(-1.0);
const int maxn = (long long int)2e9 + 5;
int main() {
  double r;
  scanf("%llf", &r);
  printf("%.15llf", 2.0 * pi * r);
  return 0;
}
