#include<bits/stdc++.h>
using namespace std;

#define minv(v) *min_element(v.begin(),v.end())
#define maxv(v) *max_element(v.begin(),v.end())
#define unq(v) sort(v.begin(),v.end()),v.erase(unique(v.begin(),v.end()),v.end())
#define ll long long
#define pb push_back
#define fill(arr) memset(arr,0,sizeof(arr))
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*(b/gcd(a,b)))
#define pi acos(-1.00)
#define _ios ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);

int main()
{
    _ios;
    ll r;
    cin>>r;
    cout<<2*pi*r<<endl;
    return 0;
}

