// God put a smile upon your face <3

#include <bits/stdc++.h>

#define slld(longvalue) scanf("%lld", &longvalue)

#define ll long long
#define ull unsigned long long
#define pll pair < long long, long long >

#define fastio ios_base:: sync_with_stdio(false); cin.tie(0); cout.tie(0)

#define pb push_back

#define bug printf("BUG\n")

#define mxlld LLONG_MAX
#define mnlld -LLONG_MAX

#define mxd 2e8
#define mnd -2e8

#define pi 3.14159265359

using namespace std;

bool check(ll n, ll pos)
{
	return n & (1LL << pos);
}

ll Set(ll n, ll pos)
{
	return n = n | (1LL << pos);
}

int main()
{
    ll i, j, k, l, m, n, o, r, q;
    ll testcase;
    ll input, flag, tag, ans;

//    freopen("input.txt", "r", stdin);

//    freopen("output.txt", "w", stdout);

    double rad;

    while(cin >> rad)
    {
		cout << setprecision(10) << fixed << 2.0 * acos(-1.0) * rad << "\n";
    }


}



#include<bits/stdc++.h>
#define rep(i,n) for (int i=0; i<n; ++i) // マクロ
using namespace std; // stdの省略
using pii = pair<int, int>; // pairの略記
using ll = long long; // long longの略記


int main(void) {
   
   int r;
   cin >> r;
   
   
   
   cout << 2*r*M_PI << endl;
   
   return 0;
}