#include <bits/stdc++.h>

using namespace std;

#define LL long long
#define ULL unsigned LL
#define LD long double
#define debug(x) cerr << #x << " " << x << endl;

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	LD r;
	cin>>r;
	LD w = 2.0 * M_PI * r;
	cout<<setprecision(10)<<fixed<<w;
}
