#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int N = 1e5 + 5, mod = 1e9 + 7;


int main(){
    ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
    long double r;
    cin >> r;
    cout << fixed << setprecision(10) << 2.00 * r * 3.14159265359;

    return 0;

}


