#include <bits/stdc++.h>

typedef long long LL;
typedef unsigned long long LONG;
const int N = 1e5 + 5;
const int INF = 0x3f3f3f3f;
const int MAXINT = 2147483647;
const int mod = 1000000007;
const long long MAXLONG = 9223372036854775807;
using namespace std;
typedef pair<int, int> PII;
#define ft fisrt
#define sd second
#define eb emplace_back
#define END '\n'

template <class T>
inline bool Read(T &ret)
{
	char c;
	int sgn;
	if (static_cast<void>(c = getchar()), c == EOF)
		return 0;
	while (c != '-' && (c < '0' || c > '9'))
		c = getchar();
	sgn = (c == '-') ? -1 : 1;
	ret = (c == '-') ? 0 : (c - '0');
	while (static_cast<void>(c = getchar()), c >= '0' && c <= '9')
		ret = ret * 10 + (c - '0');
	ret *= sgn;
	return 1;
}

template <class T>
inline void Write(T x)
{
	if (x < 0)
		putchar('-'), Write(x);
	if (x > 9)
		Write(x / 10);
	putchar(x % 10 + '0');
}

void solve(int question_num)
{
	while(question_num--){
		int n;cin>>n;
		cout<<n*2*3.1415926;
	}
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int t = 1;
	// cin >> t;
	solve(t);
	return 0;
}
