#include<bits/stdc++.h>
#define fo(i,a,b) for((i)=(a);i<=(b);i++)
#define rfo(i,a,b) for((i)=(a);i>=(b);i--)
#define inrange(x,y,z) (((x)>=(y))&&((x)<=(z)))
#define ALL(vec) ((vec).begin(),(vec).end())
#define SOR(vec) sort(ALL(vec))
#define UNI(vec) (vec).erase(unique(ALL(vec)),(vec).end())
using namespace std;
long double r,pi;
int main(){
	#ifdef FILIN
		#ifndef LOCAL
			freopen(FILIN,"r",stdin);
			freopen(FILOUT,"w",stdout);
		#endif
	#endif
	ios::sync_with_stdio(0);
	cin.tie(NULL);
	cout.tie(NULL);
	cin>>r;r*=2;
	pi=3.1415926535897932384626433832795;
	cout<<fixed<<setprecision(90)<<r*pi<<endl;
	return 0;
}
