#include<bits/stdc++.h>
#define ll long long
#define pb push_back
using namespace std;
int main()
{
    string a, b, s;
    double n, m, k, r, pi = 3.1416, x = 2.0;
    cin>>r;
    printf("%.10f", x* pi * r);
	return 0;
}
