#include<bits/stdc++.h>
#define fi        first
#define se        second
#define pb        push_back
#define PI        acos(-1)
#define LC(a)     ((a<<1))
#define RC(a)     ((a<<1)+1)
#define MID(a,b)  ((a+b)>>1)
#define mem(a, b) memset(a, b, sizeof(a))
#define IOS()     std::ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
using namespace std;
typedef long long ll;
typedef unsigned long long ULL;
typedef pair<int,int> PII;
typedef pair<ll,ll>   PLL;
typedef pair<ll,int>  PLI;
const int INF = 0X3F3F3F3F;
const int MIN = -(1<<30);
const ll N = 1e5+7;
inline void solve()
{
	double n;cin>>n;
	printf("%.6lf\n",n*PI*2);

}
int main()
{
	solve();
	return 0;
}


