#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>;

const int MOD = 1000000007;
const int INF = 999999999;

int main(){
   int r;
   cin >> r;
   cout << 2*r*acos(-1) << endl;
return 0;}