
// Problem : A - Circle Pond
// Contest : AtCoder - AtCoder Beginner Contest 163
// URL : https://atcoder.jp/contests/abc163/tasks/abc163_a
// Memory Limit : 1024 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)

#pragma GCC optimize("O3")  
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define pii pair<int,int>
#define S second
#define endl "\n"
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*b)/gcd(a,b)
#define mem(z,i) memset(z,i,sizeof(z))
#define eps 1e-7
#define I (int)
#define mod 1000000007
#define YES "YES"
#define NO "NO"
const long long INF = 1000000000000000000LL;
//const int WASTE=∞;
ll power(ll a,ll b){
	ll i=1;
	while(b){
		if(b%2)
			i=(i*a)%mod;
		b/=2;
		a=(a*a)%mod;
	}
	return i;
}
ll mulmod(ll a, ll b) { 
    ll res = 0;
    a = a % mod; 
    while (b > 0) {
        if (b % 2 == 1) 
            res = (res + a) % mod; 
        a = (a * 2) % mod; 
        b /= 2; 
    }
    return res % mod; 
} 
const int N=1e5+5;
signed main() {
    ios_base::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
  	cout<<fixed;
  	cout.precision(10);
   	int b;
   	cin>>b;
   	double ans=b*2*3.14;
   	cout<<ans;
   	
   	
   	
   	
   	
   	
   	
   	
   	
   	
   	
  	return 0;
}

