#include<bits/stdc++.h>
#define rep0(i,n) for(i=0;i<n;i++)
#define rep(i,n) for(i=1;i<=n;i++)
#define reps(i,a,n) for(i=a;i<=n;i++)
#define mem(ara,n) memset(ara,n,sizeof(ara))
#define memb(ara) memset(ara,false,sizeof(ara))
#define all(x) (x).begin(),(x).end()
#define fast ios_base:: sync_with_stdio( false ); cin.tie(0); cout.tie(0);
#define eb              emplace_back
#define em              emplace
#define pb              push_back
#define Mp              make_pair
#define ff              first
#define ss              second
#define mod 1000000007
# define M_PI          3.141592653589793238462643383279502884L
typedef long long ll;
using namespace std;

const int M = (int)(2e6 + 239);

int getnum(string a)
{
    int i,p=0;
    rep0(i,a.size())
    {
        p*=10;
        p+=a[i]-'0';
    }
    return p;
}

int main()
{
    /*#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #endif*/
    fast

    double a;
    cin>>a;
    a = 2*M_PI*a;
    printf("%.10f\n",a);
}
