#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cstdlib>  
#include<cmath>   
#include<cctype>
#include<string>
#include<set>
#include<iomanip>
#include<map>
#include<algorithm>
#include<functional>
#include<vector>
#include<climits>
#include<stack>
#include<queue>
#include<list>
#include<deque>
#include<climits>
#include<typeinfo>
#include<utility>
#define LEN 100005
#define rep(i,n) for(int i=0;i<n;i++)
#define Rep(i,n) for(ll i=0;i<n;i++)
#define For(i,m,n) for(int i=m;i<n;i++)
#define FOR(i,m,n) for(ll i=m;i<n;i++)
using namespace std;
using ll = long long;
double pi() { return 3.14; }
int sum(int a, int b) { return a + b; }
int mul(int a, int b) { return a * b; }
ll LGCD(ll x, ll y) { if (x % y == 0) return y;else return LGCD(y, x % y); }
ll LLCM(ll x, ll y) { return x * y / LGCD(x, y); }

int GCD(int x, int y) { if (x % y == 0) return y;else return GCD(y, x % y); }
int LCM(int x, int y) { return x * y / GCD(x, y); }
ll dpc[10000][10000];

//void nCr(ll n, ll r) {
//	ll ans = 1;
//	for (int i = n; i > n - r; --i) {
//		ans = ans * i;
//	}
//	for (int i = 1; i < r + 1; ++i) {
//		ans = ans / i;
//	}
//	printf("%d",ans);
//}

ll combination(ll n, ll r) {
	if (n == r) { return dpc[n][n] = 1; }
	else if (r == 0) { return dpc[n][0] = 1; }
	else if (r == 1) { return dpc[n][1] = n; }
	else if (dpc[n][r]) { return dpc[n][r]; }
	else { return dpc[n][r] = combination(n - 1, r - 1) + combination(n - 1, r); }
}

//cout << fixed << setprecision(10);


int main() {

	int r;
	double l;
	cin >> r;
	l = r * 2 * 3.1415926535;
	cout << l << endl;



	

	return 0;
}
