#include <bits/stdc++.h>
#define MAX_INT  ((unsigned)(-1)>>1)
#define MIN_INT  (~MAX_INT)
#define db printf("where!\n");
#define pb push_back
using namespace std;
#define ll long long
ll gcd(ll x,ll y){return y ? gcd(y,x%y) : x;}
template<class T>inline void read(T &res){
char c;T flag=1;
while((c=getchar())<'0'||c>'9')if(c=='-')flag=-1;res=c-'0';
while((c=getchar())>='0'&&c<='9')res=res*10+c-'0';res*=flag;
}
double pi=3.1415926;

int main(){
	double r;cin>>r;
	printf("%.2f",r*2*pi);
	return 0;
}