#include<bits/stdc++.h>
#define ll long long
#define pb push_back
using namespace std;
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    double r;
    cin>>r;
    double pi=3.1415926;
    pi=2*pi*r;
    cout<<pi<<endl;
    return 0;
}
