#include <bits/stdc++.h>
using namespace std;
#pragma warning(disable : 4996)
#define gc getchar_unlocked
#define fo(i, n) for (i = 0; i < n; i++)
#define ll long long
#define si(x) scanf("%d", &x)
#define sl(x) scanf("%I64d", &x)
#define ss(s) scanf("%s", s)
#define pb push_back
#define em emplace_back
#define mp make_pair
#define F first
#define S second
#define endl '\n'
#define clr(x) memset(x, 0, sizeof(x))
#define PI 3.1415926535897932384626
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<pii> vpii;
typedef vector<pll> vpll;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
const int mod = 1e9 + 7;
int epow(int base, int pow)
{
    int result = 1;
    while (pow)
    {
        if (!(pow % 2))
        {
            base = (base * base) % mod;
            pow /= 2;
        }
        else
        {
            result = (result * base) % mod;
            pow--;
        }
    }
    return result;
}
int main(void)
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n;
    si(n);
    double d = 2 * n * PI;
    cout << d;
}