#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define pb push_back
#define mp make_pair
#define fast ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
#define ff first
#define ss second
#define INF (ll) 1e9
#define endl "\n"
#define tt ll testcase; cin>>testcase; while(testcase--)
 
int main(){
	fast;
	ll n;
	cin>>n;
	double ans = 2*((double)n)*(3.14159);
	printf("%.4lf", ans);
}