#include<bits/stdc++.h>
#define pf printf
#define sc(x) scanf("%d",&x);
#define scs(x) scanf("%s",x);
#define scl(x) scanf("%lld",&x);
#define rep(i,s,e) for(int i=s; i<=e; ++i)
#define dep(i,s,e) for(int i=s; i>=e; --i)
typedef long long ll;
using namespace std;
const ll mod=998244353;
const double pi=acos(-1); 
int main()
{
	int r;
	sc(r);
	double cou=2.0*r*pi;
	pf("%0.30lf\n",cou);
}
