#include<bits/stdc++.h>

using namespace std;

#define ll			long long
#define ALL(s)			(s).begin(),(s).end()
#define ALLn(s,n)		s,s+n
#define F			first
#define S			second
#define pb			push_back
#define tc(t)			int t; cin >> t; while(t--)
#define _			ios_base::sync_with_stdio(false); cin.tie(NULL);
#define PI 3.141592653589793238
const ll MOD = 1e9 + 7;
const ll INF = INT_MAX;
const ll MAX_N = 1e6 + 7;


void solve() {
    int r;
    cin >> r;
    cout << fixed << setprecision(20) << 2*PI*r << "\n";
}

int32_t main() {_
    //tc(t) 
	solve();
}
