#include<bits/stdc++.h>
using namespace std;
#define pb			push_back
#define vi  		vector<int>
#define all(a)		(a).begin(),(a).end()
#define F 			first
#define S 			second
#define endl		"\n"
#define fast 		ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define int 		long long int
#define mod			1000000007

static int t[3][4001];
int32_t main(){


	int r;
	cin>>r;
	float ans=2*3.141592*r;
	printf("%.2f",ans);
	return 0;
}