#include<bits/stdc++.h>
#define ll long long
using namespace std;
#define mk  make_pair
#define pb  push_back
#define pi 3.141592653589793238

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);

    double n;
    cin>>n;
    double ans=2*pi*n;
    cout<<fixed<<setprecision(20)<<ans<<endl;
    return 0;
}
