#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define mod 1000000007

int main() {

    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);

    ll t = 1;
    //cin>>t;

    while(t--) {
        double r;
        cin>>r;

        double ans = 2 * r * (3.14159);
        cout<<ans<<"\n";

    }

}

