#include <bits/stdc++.h>
#pragma GCC optimize(2)
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC optimize(3)
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC target("sse3", "sse2", "sse")
#pragma GCC target("avx", "sse4", "sse4.1", "sse4.2", "ssse3")
#pragma GCC target("f16c")
#pragma GCC optimize("inline", "fast-math", "unroll-loops", \
                     "no-stack-protector")
using namespace std;
const int N = 10010;
const int M = 150;
const int INF = 0x3f3f3f3f;
const int MOD = 998244353;
const double PI = 3.1415926;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ;
  int r;
  cin >> r;
  cout << PI * 2 * r << endl;
  return 0;
}
