#include<bits/stdc++.h>
#define ll long long
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
using namespace std;
 
int main(){

    float r;
    cin >> r;
    float ans;

    ans = 2*r*3.14159;

    cout << ans;

	return 0;
}