#include<bits/stdc++.h>
//#define int long long
#define rep(i, l, r) for(register int i = l; i <= r; ++ i)
using namespace std;
const int N = 100010, mod = 2147483647;
const double PI=3.1415926;
signed main()
{

	// freopen("testdata.in","r",stdin);
	// freopen("testdata.out","w",stdout);
	int n;
	//double n;

	scanf("%d",&n);

	printf("%.2lf",2 * (double) n * PI);
	return 0;

}