#include <bits/stdc++.h>
    using namespace std;
    typedef long long int ll;
     
    int main() {
    	#ifndef ONLINE_JUDGE
        // for getting input from input.txt
        freopen("input.txt", "r", stdin);
        // for writing output to output.txt
        freopen("output.txt", "w", stdout);
        #endif
     
        float c;
        int r;

        cin>>r;

        c = float(22 * 2 * r)/float(7);
        cout<<c;
   
        return 0;
    }