#include <bits/stdc++.h>

using namespace std;

#define fastio cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(0)
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vector<int>> matrix;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define all(c) c.begin(),c.end()
#define mp make_pair
#define pb push_back
#define f first
#define int ll
#define tcase int t; cin>> t; for( ; t; --t)
#define s second
#define endl '\n'
const int MAX = 1e6+1;
const int INF = 0x3f3f3f3f;
const double PI = acos(-1);
double n;

int32_t main(){
	fastio;
	cin>> n;
	cout<< 2*PI*n<< endl;
	return 0;
}
