/* Author - xorier4104 */

#include <bits/stdc++.h>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>


using namespace std;

#define ll              long long int
#define ld              long double
#define db              double
#define pi              3.1415926535897932384626
#define PI              acos(-1.0)
#define endl            "\n"
#define mod             1000000007
#define MOD 			(int) 1e9+7
#define mk              make_pair
#define pb              push_back
#define pii             pair<int,int>
#define vi              vector<int>
#define mii             map<int,int>
#define pqb             priority_queue<int>
#define pqs             priority_queue<int,vi,greater<int> >
#define setbits(x)      __builtin_popcountll(x)
#define zrobits(x)      __builtin_ctzll(x)
#define ps(x,y)         fixed<<setprecision(y)<<x
#define w(t)            ll t; cin>>t; while(t--)
#define sc(a)	        scanf("%lld",&a);
#define pr(a)	        printf("%lld\n",a)
#define f(i,a,n)		for(ll i=a;i<n;i++)
#define m(a)		    memset(a,0,sizeof(a))
#define mem(a,b)        memset(a,(b),sizeof(a))
#define max(a,b)        ((a)>(b) ? (a):(b))
#define min(a,b)        ((a)<(b) ? (a):(b))
#define abs(a)          ((a)>0 ? (a):(-(a)))
#define sq(a) 			((a)*(a))
#define FastIO          ios_base::sync_with_stdio(0);cin.tie(0),cout.tie(0)

int main(){ FastIO;

	int n;

		cin>>n;
		double answer=2*n*pi;

		cout<<setprecision(20)<<answer<<endl;

return 0;	
}