//OPTIMIZATONS
#pragma GCC optimize("02")
//CODE
#include<bits/stdc++.h>
#define int long long
#define float double
#define endl "\n"
#define debug cout<<"Good uptill here"<<endl<<flush;
#define deb(x) cout<<#x<<":"<<x<<endl<<flush;

using namespace std;

int32_t main(){
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	//CODE FOR SPECIFIED INPUT AND OUTPUT
	//freopen("input.txt", "r", stdin);
	//freopen("output.txt", "w", stdout);
	double n;
	cin>>n;
	cout<<fixed<<setprecision(10)<<M_PI*n*2<<endl;
	return 0;
}

