#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int inf=0x3f3f3f3f;
const int MOD=1e9+7;
const int maxn=1e5+10;
ll gcd(ll a, ll b){return (b==0)?a:gcd(b, a%b);}
bool isPrime(ll n){
    if(n<=3){
        return n>1;
    }
    if(n%6!=1&&n%6!=5){
        return false;
    }
    ll sqr=sqrt(n);
    for(ll i=5;i<=sqr;i+=6){
        if(n%i==0||n%(i+2)==0){
            return false;
        }
    }
    return true;
}
int dis[4][2]={0,1,0,-1,1,0,-1,0};
int read(){
    int x=0,f=1;char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
    return x*f;
}


int main()
{
	int n;cin>>n;
	float pi=3.1415926535898;
	printf("%lf\n",pi*2*n);
}
 