#include <bits/stdc++.h>
using namespace std;
#define M 1000000007
#define ll long long
#define fo(i,n) for(int i=0;i<=n;i++)
int main()
{
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif
	int t, n, c;
	cin >> t;
	cout << (2 * 3.141 * t);
}